Fanon/Literature/Harry Potter

The Harry Potter books, of course, have their fair share:
 * 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.
 * 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.