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Because this chapter's title refers to “mistakes,” I want to stress that this chapter isn’t about ruining your fun. There is a great difference between “writing something that is at odds with canon (or history, or British culture) because you don’t know any better” and “writing something that is at odds with those things because you deliberately chose to write your story that way.” It is one thing to break the rules of writing by accident and another thing to break them on purpose, and adherence to canon is different in only one respect: you don’t necessarily have to refer to a rule of writing before you break it, but it is generally wise to refer to a departure from canon (or even widely-held fanon) before that departure becomes important to the plot.
This shouldn’t need saying, but Pure-bloods are not specialer or better at magic than Muggle-borns or Half-bloods. Pure-blood supremacism is a dirty ideology with no more justification than any type of racial supremacism.
Don’t mistake aesthetics for values. British wixes are not a bunch of temporally-displaced Victorians. A woman became Minister for Magic in 1798, and there’s no indication of racism (far from it, in fact, since Cantankerus Nott seemed to have no problem listing the Shacklebolts and Shafiqs in the Pure-blood Directory ).
It’s also worth noting that, while boys can’t enter a girl’s bedroom and there are separate lavatories for boys and girls, the Quidditch changing rooms appear to be unisex, as demonstrated by this passage from COS:
The rest of the Gryffindor team were already in the changing room. Wood was the only person who looked truly awake. Fred and George Weasley were sitting, puffy-eyed and tousle-haired, next to fourth year Alicia Spinnet, who seemed to be nodding off against the wall behind her. Her fellow Chasers, Katie Bell and Angelina Johnson, were yawning side by side opposite them.
The Death Eaters are not going to turn on Voldemort because they found out that he’s a Half-Blood. They know he’s a Half-blood already, and we know that they know this because we see Voldemort talk about his Muggle father and his father’s grave in a very Muggle graveyard in GOF. This doesn’t rule out the possibility that the Death Eaters are in denial about it all and slamming shots of Firewhiskey behind the scenes whenever they think about it (Bellatrix is 110% in denial), but either way, it isn’t going to be news to them when Harry Potter starts dropping pamphlets in Diagon Alley or whatever. For Merlin’s sake, when did “the facts” ever convince the fans of your least favorite politician that they were a berk? Don’t expect wizard fascists to have a higher epistemic standard.
The Wizengamot is probably not hereditary — Ralston Potter and Henry Potter are the only Potters mentioned as being members, and we know enough about Fleamont and James (to say nothing of Harry) that we can expect to have heard about any seats on the Wizengamot that they might’ve had. The Sacred Twenty-eight are not a Who’s Who of the Wizengamot — they’re a list of families assembled in the early 20th century by a single bigot, who specifically included or excluded some families because of how he personally felt. Also, Lucius Malfoy is not a member of the Wizengamot.
Pure-bloods are more aware of the Muggle world than we often give them credit for. Draco Malfoy certainly couldn’t give you the technical specs for a helicopter, but even at the age of eleven, he did know that they were some sort of flying device and that they were quick enough that “narrowly escaping” one would be brag-worthy rather than embarrassing; and Kingsley Shacklebolt infiltrated the Muggle British government so well that he was regarded as one of the best members of the Prime Minister’s staff in 1996, even though Prime Minister already knew about magic.
It’s worth pointing out that the Fat Friar exists, so — setting aside any arguments about the compatibility of magic and Christianity (see below for that) — there has been at least one (1) wixen Christian in all of history. This holds little import for determining how many wixen Christians there were or are, but I’ve occasionally seen people posit that there have never been any.
The Ministry (thinks that they) found more of Pettigrew than just his finger. The confusion may be due to a difference between the film and book versions of POA. In the film, it’s stated that a finger was “all that was left,” but in the book, we’re told that a finger was “the biggest bit of him they could find,” meaning that there were other bits that (the Ministry believed) belonged to Pettigrew. Considering the explosion and the deaths of twelve Muggles, the scene was probably a little splatterpunk and the Ministry didn’t bother to check which people-bits belonged to whom.
Gringotts is not the same thing as the “goblin nation” (which, as a term, does not show up in the books). The goblin rebellions do not imply that Gringotts is a nation any more than the American revolution implies that JPMorgan is a nation. It is also unclear whether there is a goblin state at all: the fact that there were goblin rebellions suggests that the Ministry of Magic was a dominant power, and it is plausible (though not certain or even probable) that there is no “goblin government” that is recognized by the Ministry or the I.C.W., as the Servile Wars do not imply that the slaves of Rome were a nation themselves, or constituted a government.
When Herbert Chorley, a junior minister in the Muggle government, was subjected to a “poorly performed Imperius Curse,” the Prime Minister did not suspect magic, even though he knew that magic existed.
There is no restriction on the use of magic for wixes that have finished their term in Azkaban and returned to free society.
Aurors and Hit Wizards are probably the same thing (Hit Wizards are mentioned only in POA, and Aurors are mentioned only from GOF on).
Harry, Ron, and Hermione are never described as “the Golden Trio.” Even the word “trio” never shows up except for a single reference to the Deathly Hallows.
We don’t know whether Regulus Black was a “good guy” at the end or not. He might’ve made a complete heel-face turn, or he could have just been a Pure-blood bigot who realized that Voldemort wasn’t a selfless crusader for Pure-blood rights. Yes, he cared a lot for Kreacher, but the same appears to be true of Bellatrix Black.
Susan Bones lost an uncle, an aunt, and multiple cousins to Death Eaters, but her parents may still be alive. She probably doesn’t live with her aunt Amelia, because she was alive even after the Death Eaters killed Amelia, but she could have easily been with a friend or something at the time.
Albus Dumbledore wasn’t certain that Voldemort had made a Horcrux until he had the Diary, and was only sure about multiple Horcruxes after Voldemort’s resurrection, though the state of the Diary made multiple Horcruxes probable. Dumbledore also hoped that Harry would survive: the “gleam of something like triumph in Dumbledore’s eyes” in GOF comes directly after Dumbledore learns that Voldemort’s resurrection made use of Harry’s blood, which was instrumental in keeping Harry alive. Also, he was very unconcerned with the prophecy, except to the extent that Voldemort's obsession with it made Voldemort more manageable. He only offered someone a sherbet lemon (or lemon drop, in American editions) once, and it was to Professor McGonagall.
Bellatrix Black may revere Voldemort but she doesn’t accept his opinions uncritically, e.g. saying that “The Dark Lord is… I believe… mistaken” to trust Snape (though she is only willing to say so explicitly to Narcissa, and remains silent when Snape himself asks this).
Sirius Black probably has a middle name, since his brother does, and it is very plausible that his middle name is Orion, after his father, just as his brother’s middle name is Arcturus, after their grandfather. Nevertheless, we don’t know either of these for a fact, so feel free to do what you want and still call the fic “canon compliant.”
Cornelius Fudge believed that Voldemort could “rise again” if Sirius Black found him, so the idea that Voldemort was still around in some sense was not unimaginable.
Nobody refers to Hermione Granger as ‘Mione. She knows how to fly a broomstick. Her parents are not necessarily named Dan and Emma. She's kind of socially inadept in her first year at Hogwarts, but outgrows that over time.
Luna Lovegood is neither a Seer nor an untrained Legilimens. The primary difference between Luna Lovegood and Alex Jones is a pleasanter disposition (well, that and the genuineness of their beliefs).
Remus Lupin is not obsessed with chocolate.
Abraxas Malfoy was not necessarily a Death Eater, and did not necessarily go to school with Tom Riddle.
Harry Potter is not called “cub” or “pup” or anything similar, nor is he called anybody's "golden boy." His grandparents are Fleamont and Euphemia, not Charlus and Dorea. He does not have "Avada Kedavra orbs." He was excited about the Triwizard Tournament and even prospect of being the Hogwarts champion (until it happened, anyway). As DeepSpaceCraft points out, Harry also says, more or less and in front of witnesses, “If I were going to put my name in the Goblet of Fire then I’d do it when nobody was watching,” so there’s a bit of justification for other students to think that maybe, just maybe, he did just that.
James Potter was Head Boy and Chaser on the Gryffindor Quidditch team, but he is not specifically stated to have either been a prefect or Quidditch captain.
Severus Snape is not Draco Malfoy’s godfather, or at least he isn’t said to be. For the record, nobody is anybody’s godfather except for Sirius Black (Harry Potter) and Harry himself (Ted Lupin); but Bill Weasley appears to be familiar with the concept, so...
Arthur Weasley’s ignorance of many Muggle things is not so much a problem as we might think. It’s true that he handles Muggle artifacts, but this doesn’t necessarily mean that he directly interacts with Muggles, except in situations where the Muggles are going to be Obliviated later. He doesn’t need to know how to speak on a telephone, only that Muggle telephones aren’t supposed to bite your ear off, and given that it’s possible to detect enchantments he may not even need to know that much. Kingsley Shacklebolt on the other hand was able to operate undetected on the Prime Minister’s staff, and the Muggle Liaison Office can be expected to have greater familiarity with the Muggle world, and even they might not know exactly how an electric plug works. There are plenty of Muggles that don’t know.
It also isn't clear how much of his ignorance is just for show. When the subject comes up with a fellow adult at his workplace, rather than with some kid, Arthur demonstrates a familiarity with the technical vocabulary:
Then he said in normal tones, “And don’t take too long, Weasley, the delay on that firelegs report held our investigation up for a month.”
“If you had read my report you would know that the term is ‘firearms,’ ” said Mr. Weasley coolly. “And I’m afraid you’ll have to wait for information on motorcycles, we’re extremely busy at the moment.”
We can't be sure whether Kingsley feigned ignorance for the sake of their pretend conversation (if so, he must be a quick study in order to, as mentioned above, fool even the Prime Minister), but Arthur clearly knows what he's talking about.
Also worth noting vis-à-vis “Tell me, what exactly is the function of a rubber duck?”:
(1) Arthur only asks this question in the film adaptation of COS.
(2) On 10 January 1992, more than 20,000 rubber ducks were dumped overboard from a cargo ship in the Pacific Ocean, and from that time the ducks have been used to track ocean currents and confirm and refine existing models. Harry doesn’t show up at the Burrow until July 1992, so Arthur very well might have heard about this by the time that he asks Harry what’s up with rubber ducks.
Fred and George Weasley are probably the only Weasley twins in living memory, and there is no indication that Fabian and Gideon Prewett were twins. Fraternal twins run in families, but identical twins do not (this might be different where magic is involved, though). They only made the “Gred and Forge” joke a single time.
Molly Weasley did not necessarily dose Arthur with a love potion and, if she ever had done so, the love potion was not necessarily Amortentia. There are many kinds of love potion, of which Amortentia is the strongest, not the most typical. If there were anything shady about what Molly had done, she probably wouldn't be telling the story at the breakfast table. She might have used the love potion on someone else, the use of the love potion might have been consensual (there are plenty of reasons that you might be willing to drink a love potion), and the love potion may have had relatively minor effects (the love potion which Ron is accidentally dosed with, for example, is no more severe than a Confundus Charm, and there's nothing to suggest that Ron would have done anything more than "act very silly as he proclaims his love for Romilda").
Percy Weasley is not named “Percival.”
If you hate Ron Weasley and you haven’t read the books in a few years, then I strongly encourage you to reread them. That might not change your position, but it’s a fact that (1) the films portray him in a worse light than the books and (2) the films sometimes influence our sense of canon more than we realize (just think about how many people talk about Arthur Weasley’s “rubber duck obsession” as if this is something that ever comes up in the books). He’s certainly not dumb, since he got as many O.W.L.s as Harry did.
Some observations on the relative heights of certain characters:
Therefore, by Deathly Hallows, Draco is tall, Harry is probably tall but not so much as Draco, Fred and George may be a little taller than Draco, and Ron is the tallest of them all. There may be an inconsistency between "Fred and George are short and stocky" and "Fred and George are taller than Harry," since Harry can't be short if Draco is both "tall" and only "slightly taller" than Harry, but maybe wizards are just built different. Fleur is also about as tall as Harry, because she neither shoots upward nor shrinks when she takes the Polyjuice.
Hogwarts letters are sent in July, not on the student’s birthday. Letters were being sent to Harry days before his birthday, but he finally read his letter on his birthday because that’s when Hagrid arrived and forced the issue. In fact, the cut-off date for a reply is on Harry’s birthday.
The school uniform is robes (black) and a pointed hat (also black, for day wear), and a winter cloak (black, with silver fastenings), and all clothes are supposed to carry name tags. There is no mention of neckties, nor anything that might portray a student’s house in the normal course of things. Quidditch team robes are colored differently, but it’s a tiny plot point in COS that, while Polyjuiced, Harry and Ron mistakenly ask a Ravenclaw for directions to the Slytherin common room.
“Charms Master,” “Potions Master,” and so forth do not refer to a “mastery” in the sense that a Master’s degree refers to an additional educational attainment. It is simply a(n old-fashioned) way of saying “Charms teacher,” “Potions teacher,” etc. John Nettleship, for example, was Rowling’s Chemistry master.
Minerva McGonagall doesn’t give welcoming speeches to new Gryffindors. Whether the other Heads do anything to welcome new students is unknown.
“O,” “E,” and other letter grades are unique to the O.W.L. and N.E.W.T. tests.
The Yule Ball was a special dance that occurs in tandem with the Triwizard Tournament, and not an annual event. The Magic Awakened game says that the Yule Ball was popular enough that it inspired the scheduling of annual dances in years to come, which is a fun idea, but you can take that or leave it as you please.
In terms of comparing magic to Muggle science, it’s better to think of wixes as “differently sophisticated” rather than “less sophisticated.” Technology doesn’t advance through discrete tech levels which manifest everywhere in the same way, but rather adapts to the culture which uses it (just as the culture adapts to knock-on effects of that technology). Rather than saying, “Wixes don’t have cell phones, so they’re technologically backwards,” we ought to remember that a fifteen-year-old student once built a pager network out of magical coins and two kids in the 1980s owned a pair of looking-glass mobile videophones more than a decade before Muggles had anything similar, and conclude that maybe Magical Britain just doesn’t care much about that shit (perhaps the ease of personal and fireplace-based teleportation means that they continue to prefer in-person interaction).
The effect of magic on Muggle technology is often overblown. This idea is founded in Hermione’s comment in GOF that “all those substitutes for magic Muggles use — electricity, computers, and radar, and all those things — they all go haywire around Hogwarts, there’s too much magic in the air.” First, all three of these rely on the electromagnetic spectrum, so things like “plastic” and “combustion engines” and “vaccines” will still work just fine. Second, if wixes themselves produced an appreciable anti-tech field then Hermione would certainly have mentioned that, because she’s explaining why Rita Skeeter couldn’t have gotten her information from a Muggle listening device, and it makes no sense to talk about Hogwarts if Rita’s victims would have fucked with her hypothetical bug all by themselves. While we’re on the subject of combustion engines, by the way, I’d like to mention that while that the engine, specifically, has naught to do with electricity, the spark plug does, so a Muggle car would probably be able to drive by Hogwarts but might have some trouble starting up nearby Hogwarts).
You do not need to have visited a location in order to Apparate there, though a prior visit or even a photograph will probably assist in the “Destination” element of the three D’s.
Transfiguration is generally permanent, or at least can be made permanent by a competent wix and not just an expert one (this statement has nothing to do with the possibility of performing an untransfiguration).
There is no method to permanently block, remove, or suppress a wix’s magic, which is a major reason why Azkaban is patrolled by dementors and why wixes can break out of Azkaban and subsequently cause trouble.
The Killing Curse can be blocked by at least some physical objects, as when it “merely glanced off” the chest of a golden statue. Perhaps the gold of the statue, or its animation, or some other magical quality, was responsible, but something happened.
“The Common Misconception of the HBP Boat Scene (Or, Why Magical Power and Magical Cores Aren’t Canon),” by Lunanight, is exactly what it sounds like. So much debate about magical cores comes down to the fucking Boat Scene.
There is Dark magic (and there are Dark Arts), but no such thing as Light magic (nor Light Arts).
Pepper-up Potion treats colds, not fatigue (though they might help fatigue to whatever degree that “getting a bit warmer” will or won’t help).
Animagus forms and Patronuses don’t have to match, and probably don’t most of the time.
Wixes have pets, not familiars.
The incompetence of adult wixes in re Shield Charms is probably overstated. That is to say, they probably aren’t good at Shield Charms, but we shouldn’t use that fact to draw any conclusions about their magical competency in general. Consider the following:
Given these facts, it is plausible that the Shield Charm is not taught until seventh year, or even that it is not a part of the core curriculum at all. It might receive a week or two of attention, then get occasional reinforcement in subsequent lessons but nothing substantial. Even if it is normally taught and greatly emphasized in sixth year it would be part of the N.E.W.T. curriculum, which means that only D.A.D.A. N.E.W.T. students will learn it.
It’s a common misconception that Tom Riddle experienced the Blitz, but the Blitz took place from 7 September 1940 to 11 May 1941, while Tom was safely ensconced at Hogwarts. It’s completely fine for the Blitz to have affected him in the way that writers often describe — upon his return, Tom would have been greeted with the physical evidence, hundreds of thousands of tons of rubble, and horror stories that were already transforming into legend — but he would not have personally experienced it.
Medieval European Christianity was not strongly prejudiced against wizards, and this goes double for Catholicism. The majority of witch hunts were (1) post-Middle Ages and (2) perpetrated by Protestants. Some forms of magic were banned (usually divination, or some specific variety of divination) but others were permitted.
An example of the complicated portrait of these times is the Council of Paderborn in A.D. 785, which made the act of witch-burning a crime in itself, and punishable by death (because witch-burning was witch-killing and witch-killing was murder), though it should be noted that Charlemagne didn’t think that witches existed, and there could be (less lethal) punishments for holding a belief to the contrary. The real Charlemagne was obviously correct to say that magic doesn’t actually exist, but this situation does make for a thorny problem when we try to account for the Council of Paderborn in a world where magic very much did exist. Perhaps Charlemagne disbelieved in witches in the sense of “persons who acquired their magical power through a deal with the Devil,” which is close enough to the sort of witch that he was certainly talking about in our world.
In the British Isles, witch hunts were fiercest in Scotland and almost nonexist in Ireland. Even so, it might be going too far to say that Scottish wixes all hate religion or something like that. Witch hunts rarely occur in isolation, and at the same time that the Muggle Scots were persecuting witches, they also were persecuting e.g. Catholics, so a Scottish wizard might not see anything incongruous about being both a wizard and a Catholic. In fact, because Catholicism was often being driven underground across Great Britain during the period preceding the Statute, we might expect Catholicism to be more common in magical families.
The situation is more complicated with regard to Judaism. As the saying goes, wherever there are two rabbis, you will find three opinions. Some will say that everything that is magical is an affront to God, and there are others who will say that spellcasting is no different from any other inborn art. Some people can wiggle their ears, and other people can throw fireballs (just…not on the Sabbath, please). It’s relatively certain that, whatever restrictions are in place, most or all of them go out the window if somebody’s life is at stake, but where this is the case, there must be at least one specific person, not just a general theoretical entity, whose life is to be saved. If Judaism is going to play a major role in your story then I really, really recommend that you do some research and/or find a Jewish beta reader (or two, or three). This paragraph is just to clear up some common misconceptions.
The British are not so religious as Americans, and Evangelicals and Fundamentalists are much rarer. You are especially unlikely to see anything to do with exorcisms, especially when Tom Riddle lived at Wool’s Orphanage (neither the Church of England nor the Catholic Church were keen on exorcisms at that time), and modern exorcisms are generally more sedate than Hollywood films might’ve led you to believe. They also include visits by a psychiatrist to make sure that the supposed victim of possession isn’t just in need of medical care.
Speaking of Wool’s Orphanage: Institutions like Wool’s were phased out in the 1950s and no longer exist. Sad as it is, Harry Potter cannot grow up in an orphanage à la Charles Dickens. Also the legal precedent for fostering children was established in 1926, with the Adoption of Children Act. Generally, the one fostering a child or infant would be required to be at least 25, unrelated, and a resident of the UK. And no, the one fostering the child cannot be a sole male, unless said law was updated, if the infant/child is female. Consent has to be acquired from both parties, specifically the parent of the child and the one who is adopting the child. The welfare of the child is paramount and the applicant cannot be offered payment to take in said child.
Many people make too much of Pure-blood inbreeding. The level of intermarriage that we see in most British Pure-blood families is mostly fine from a genetic health standpoint. At most, just five hundred people are needed to prevent the most deleterious effects of inbreeding, and several estimates are much lower than that. You mostly see Hapsburg-like extremes where people are consistently marrying first cousins and closer relations (children, parents, aunts and uncles). This isn’t to say that a small population that consistently practices intermarriage will get off without any side effects, though: Ashkenazi Jews and Old Order Amish both have a greater risk of acquiring Tay-Sachs disease, for example. More benign effects include the blueish skin of the Blue Fugates of Kentucky and the deafness that once was prevalent on Martha’s Vineyard. In “Certain Dark Things,” by rentachi, motion sickness (which indeed can run in families) is associated with very Pure-blooded families like the Blacks. It is worth noting a certain lack of inbreeding is responsible for the spread of hemophilia across the royal houses of Europe: the "Hemophilia B" gene mostly likely originated as a "spontaneous mutation" in either Queen Victoria or her father, and was manifested in her grandchildren and further descendants because it was a dominant gene, rather than because her relatives were marrying close cousins or anything like that.