Fanon/Literature/Harry Potter

The Harry Potter books, of course, have their fair share of fanon:
 * There was a widespread notion that Ginny's name was short for Virginia, until JKR revealed that her full first name was actually Ginevra.
 * Nowhere are Moony, Wormtail, Padfoot, and Prongs actually referred to as the Marauders. In fact, with regards to the map, they made, it's the singular: "Marauder's", not "Marauders' ".
 * Word of God claims that they referred to themselves as such, but that was after it was widespread fanon. By then it was the easiest way out.
 * A similar one is the fandom insistence that Fred and George routinely refer to themselves/each other as Gred and Forge. This was a one-off joke in the first book, when their mother made them Christmas sweaters with their initials on them. Apparently people really liked it.
 * They also usually finish each others sentences in fanfics, this happened less than a handful of times in Canon.
 * Also, Rowling has never said that Grindlewald was Dumbledore's lover, or even definitively stated that he was aware of the latter's feelings for him. Fandom made the leap from a simple "Dumbledore loved Grindlewald" to "they were shagging like bunnies".
 * It's generally taken for granted that Snape is Draco's godfather. It's stated nowhere in canon, but most people believe it to be as true as the fact that Sirius is Harry's godfather.
 * Oh, that crazy Head Boy and Girl's dormitory which only exists in fanfiction! This has been around so long that the specifics of the room have been pretty much codified. It's inside a tower. It normally contains a common room with two separate dorm rooms, one for the Head Boy and one for the Head Girl. There is always only one bathroom to ensure hilarious sexual hijinks. It's often entered through a painting with the subject of the painting and the password being something vaguely romantic. Geez, you'd think Dumbledore would have something better to do than to be The Matchmaker!
 * “Dumbledore would have been happier than anybody to think that there was a little more love in the world,” said Professor McGonagall curtly. (But yeah, he probably wouldn't go that far.)
 * "Magical Cores", which are basically power levels, show up in ninety percent of HP fanfiction. Complete fabrication on the fans part, as they are never even hinted at in canon.
 * There's also "Ancient and Noble Houses", and their magical Lord rings. The only time a character talks about them, it's Sirius Black referring to his family, and was mocking their pureblood ideals. According to their tapestry the Blacks did in fact refer to themselves in this way, but it's impossible to determine whether it was literally true or a simple display of their arrogance and mass superiority complex.
 * And even if this was a thing, the chances that somebody named after a trade, such as Smith, Butcher or especially Potter, would be part of an "Ancient and Noble House" are next to nonexistent.
 * A number of Snapefics have him referring to or thinking of his Slytherin students as his "little snakes", prompting a Heh Heh, You Said "X" response from many readers.
 * Since Hermione's parents' names are never mentioned, fanfic writers have instead invented their own. Roger and Helen crop up frequently; Dan and Emma were common for a while, dropped in popularity for a while, probably after everyone figured out how cringe-inducingly meta it was, but as of the late 2010s seem to be on an upswing again. And at least one author has Gender Flipped "Dan and Emma" to "Danielle and Emmett", presumably to buck the trend somewhat.  Then there was a wave of "Rose and Hugo" ('cause Hermione is just as creative as Harry is) and even a bit of "Wendell and Monica" (Hermione's probably clever enough not to give her parents' their real names as false names).
 * Some of the very minor student character have been used as OC Stand-Ins enough to develop fanon personalities. Daphne Greengrass is an Ice Queen, but free from pure-blood prejudice and at least slightly more sympathetic than Pansy. Tracey Davis is Daphne's best friend and is more easygoing. Padma Patil is the opposite of Parvati and possibly the second brightest student of her year after Hermione. Blaise Zabini is a suave player type. Theodore Nott is a Jerkass and, when the other Slytherins are portrayed more positively, he will be the voice of pure-blood prejudice.
 * Since the only things known about Blaise before Half Blood Prince were his name and that he was in Slytherin (from the sorting ceremony in Philosopher's Stone), many fans assumed he was a she -- and no one ever guessed he was black. Most Fem!Blaises were porcelain-skinned beauties of obvious Italian heritage.
 * Minor Slytherin character Theodore Nott is also usually portrayed in fanfic as a close friend of Draco, despite Word of God saying that he is a loner who doesn't feel the need to fit in with Draco and his cronies.
 * Theodore, when given character development, ends up either the cruelest Death Eater out of all the Slytherins, or a sympathetic character that secretly sabotages the Death Eaters from the inside.
 * Although we see enough of Pansy Parkinson to know that she's the Alpha Bitch, we know nothing about her life outside of Hogwarts. The usual fanon is that she's from a lesser pure-blood family known for sucking up to people like the Malfoys. Thus, Pansy is a social climber hoping to become a Malfoy by marriage, which fits the usual view of her character.
 * Then there's the idea that when the officiator said Bill and Fleur were "bonded for life" he meant it literally, and wizard weddings involve magic that forces the couple never to cheat on each other. Aargh.
 * In the first and third books, Gryffindor gets House Cup points for its Quidditch performance. This has lead to the common assumption that a House's Quidditch points (10 points for scoring a goal, 150 for catching the Golden Snitch) are simply added to the House points (5 points for answering a question right, etc), but the books don't confirm this.
 * Voldemort's line "there is no good and evil, there is only power, and those too weak to seek it" gets quoted a lot, as though it were the official credo of the Death Eaters or something. Which might be possible, but it's never mentioned to be the case. The line is, after all, only spoken once in the entire series.
 * Tom Riddle's orphanage was run by nuns. This fanon was completely justifiable, even likely, until you actually meet Mrs. Cole. Nevertheless, it still persists in the world of fanfiction.
 * To show that Harry knows more than he should (especially in time-travel fics) the Sorting Hat is referred to as "Adrian", even though a name for it is never mentioned in the books.
 * The notion that Professor McGonagall went to school with Tom Riddle and was an Auror during the First Wizarding War crops up frequently. Also popular is the theory that she was a champion Quidditch player (usually Beater or Chaser), that she was a notorious prankster (or a notorious Ice Queen bookworm, or sometimes, if the author is feeling clever, a notorious Ice Queen who was secretly the forerunner to the Weasley twins), and that she was best friends with Pomfrey and Sprout.
 * Another near-universal belief is that Susan Bones is a redhead with (after puberty, at least) an impossibly voluptuous figure. Her hair color is never mentioned in the books, but she does have red hair in the films, in which was played by Chris Columbus's daughter.
 * A common theme in "X Reads the Harry Potter books" (and possibly in other types of fics as well) is that Remus has an obsession (or at greatly enjoys) chocolate.
 * This probably comes from The Shoebox Project, a fic that inspired quite a lot of common Marauder tropes. In it, chocolate is randomly Remus' Trademark Favourite Food. Everyone copied it, and it caught on.
 * Actually, it's more likely from the fact that in "Prisoner of Azkaban," after Harry's first encounter with the dementors aboard the Hogwarts Express, Lupin offers him chocolate and expresses the view that chocolate can always improve one's mood. So that's more likely where it comes from.
 * Masteries, apprenticeships and other levels and certifications of magical education beyond Hogwarts are entirely a fan creation; JK Rowling has explicitly stated that there's nothing more to learn after seventh year.
 * Except that one could argue Auror training is another few years of school. Unless Hogwarts has a wand-making class, future wand makers would HAVE to find some sort of apprenticeship. There has to be some sort of schooling or training for certain jobs and positions beyond the scope of Hogwarts. You can't just say Healers get on the job training from the start. That would be BAD.
 * The snake Harry set free from the zoo in the first book later became Nagini. This was a popular fan theory before the end of the series. After the series ended, it was sort of forgotten that the theory had never actually been confirmed and it slipped into the realm of fanon. There's even an associated quote ("Yes, it’s rather funny, really, that next to no one realized the snake that Harry set free in Philosopher's Stone turned out to be Voldemort's final Horcrux, Nagini.") attributed to Rowling, but God Never Said That. It wouldn't really make sense anyway as they are clearly different species of snake. Also, the snake at the zoo had a male voice in the film and Nagini is a female.
 * And the second Fantastic Beasts film reveals that Nagini used to be a human witch during the early 20th century.
 * The Draco Trilogy either created or codified the notion that the Malfoy family home is called "Malfoy Manor". Pretty soon it became fanon that every rich pure-blood family lives in a mansion called "[last name] Manor", even as the canon contradicted this by showing that the Black family, at least, lived at Number 12, Grimmauld Place‎ (don't think this has stopped "Black Manor" from showing up in fanfiction). The name "Malfoy Manor", however, got canonized in Harry Potter and The Deathly Hallows. Whether Cassandra Claire guessed accurately or Rowling pulled a Sure Why Not is unclear. (It's worth noting, however, that The Draco Trilogy uses the terms "Malfoy Manor" and "Malfoy Mansion" interchangeably, but only the former name got picked up.)
 * References to an American wizarding school known as "Salem Witches Institute" and placed in Salem, Massachusetts, pop up frequently in fanfiction, but no American schools are mentioned in the series proper.
 * Actually, in Book 4 there are some witches at the Quidditch World Cup with a sign saying they're from the Salem Witches Institute, but the idea that that's a school is strictly fanon.
 * It's also a popular Wild Mass Guess that Wizards of Waverly Place takes place in the same universe as Harry Potter.
 * Nearly every depiction of adult!Teddy gives him blue hair, perhaps to match his mom's pink.
 * Oh, the fics where all the Marauders live together, even after James is married. One person rightly called it "Lily playing Wendy Bird to a den of Lost Boys" fic. I think we can safely assume that Sirius, Remus and Peter all had their own homes. Sirius and James lived together in canon, but only for a while right after Sirius ran away from the "Noble House of Black".
 * Speaking of Blacks, a very wide notion is that Sirius's middle name is "Orion" (after his dad). It's never revealed in actual canon.
 * It is also a common concept among fics, particularly those with Slytherin Females with Harry, that the Slytherin females are constantly raped by their male counterparts, and that Snape takes part. No evidence at all exists to this, and it worries me that people think it could happen.
 * The quill that Umbridge made Harry write with in detention in Order of the Phoenix lacks an official name, but is almost universally referred to as a blood quill in fan fiction. It has been portrayed as everything from as a recognized and banned dark artifact to an implement normally used only for signing magical contracts.  However, it wasn't until a late-2014 posting on JK Rowling's Pottermore website that it was revealed to have been a unique item of Umbridge's own creation that no one had ever seen before she started using it on the students.
 * "Spell chaining", a dueling technique in which a sequence of combat spells are arranged in order that the final wand movements for one positions the wand properly for the start of the next, allowing the caster to whip off a barrage of spells very quickly.
 * A new bit as of the late 2010s was born of the "turn to smoke and zip around" effect used in the movies instead of proper apparition (which is also seen in the movies). One author named it "fumation", and the name -- along with a slowly-gelling set of advantages and disadvantages it has compared to apparition -- is slowly percolating through the fandom.