Many Discworld fans complained when later books established that Vetinari went to the Assassins' Guild school, when previous books had established he'd invented the modern, legal Assassins' Guild. They hadn't, although they did establish he'd legalized the Thieves Guild. A continuity problem is that the first Guild master introduced in the series, in fact in the very first book, is the distinctly shabby, low-life and down-at-heel Zlorf Flanellfoot, who leads a band of equally gutter-level killers. This character is totally against the evolving perception of the Assassins' Guild being a college for gentlemen of good family, which reaches its most detailed description in Pyramids. And the events of Night Watch occuring thirty years before the Discworld "present" clash badly with the Flanellfoot-era Guild (which must be in the same timeband): here we see the college for gentlemen killers with an entirely different President, the urbane and well-bred Dr Follett. So... which of two entirely different perceptions of the Guild is "right"?
The Assassins' Guild Diary notes that Flannelfoot was one of the few "scholarship boys" to attain high office in the Guild, suggesting his era was a brief anomaly between Follett and Cruces. Presumably, the low-status killers he surrounded himself with are fellow scholarship boys, either out of fellow-feeling or, more likely, because he didn't like the "proper" assassins very much.
Phantom, Susan Kay's retelling of The Phantom of the Opera, has achieved this in some parts of the fandom, especially regarding the names of characters who went nameless in the original Gaston Leroux novel (eg. Nadir for the Persian).
Many fans of His Dark Materials assumed that having a dæmon of the same sex indicated homosexuality. This is often considered truth nowadays by most fans, and when asked about the matter Philip Pullman said that he'd never thought about it, but that he liked the idea. One wonders what a bisexual would have. A hermaphrodite dæmon?
To confuse the matter, some animals are naturally hermaphrodite.
Neatly allows all the Animorphs to be paired up by putting Marco and Ax together instead of having two odd men out from the established Jake/Cassie and Rachel/Tobias couples. Marco and Ax are also each other's best friends after Jake and Tobias and briefly live together near the end of the series.
This actually comes from the passage 1 Enoch 69:6,
And the third was named Gadreel: he it is who showed the children of men all the blows of death, and he led astray Eve, and showed [the weapons of death to the sons of men] the shield and the coat of mail, and the sword for battle, and all the weapons of death to the children of men. And from his hand they have proceeded against those who dwell on the earth from that day and for evermore.
Pretty much everyGood Omens fanfic describes Crowley as tall and lean and Aziraphale as plump and blonde, even though the most we get in the book as far as I remember is that Crowley has dark hair, good cheekbones and Cool Shades, and Aziraphale gets manicures.
A Song of Ice and Fire is rife with fanon theories, helped by the author's tendency to leave a lot of foreshadowing and subtle clues throughout the novels. One of the most widely supported theories in the fandom is that Jon Snow's parents were Rhaegar Targaryen and Lyanna Stark, instead of Ned and an unknown woman. For a good example of the arguments made on behalf of it, there's this essay.
It is also common in fanfics for the eldest sibling to Arthur and Ashara Dayne and parent to Edric Dayne to be a man named Allem.
A lot of The Chronicles of Narnia fandom seems to firmly believe that the Pevensies got married and had kids while they were in Narnia. There is one segment of the plot involving romance for any of them in the entire series, and it never comes up again. That would be the part of The Horse And his Boy where Rabadash wants to force a marriage on Susan.
Tales of the Frog Princess has loads where Garrid and his past is concerned. This is what happens when an author leaves so much of an awesome character's life up for interpretation. We have all agreed that:
Garrid had an emotionally abusive father (names tend to vary), but his mother (who is always named Lucia) loved him very much.
Furthering that, Lucia was not in love with Garrid's father. The most popular reasons for marriage are that it was arranged, or that Garrid's father was a Stalker with a Crush, and forced her to marry him.
Also, Garrid's mother was born a human, but his father turned her into a vampire when they argued. Bit of a nasty shock for her...
Also, she died young, when Garrid was a teen.
Garrid's best friends are Andrea"Andy" Blackskull and Benjamin "Ben" Toumbclaw. Ben and Andy have a thing for each other.
Garrid had a love for mischief and is also a Deadpan Snarker. Especially where Eadric is concerned.
...but he's totally sweet to Li'l. This isn't that far from Canon, really...
He met Li'l at the age of 19.
He ran away from home, due to his father not wanting him to marry Li'l.
His surname is Finnegan.
The Hatter's famous riddle from Alice in Wonderland — "Why is a raven like a writing desk?" — was intended, according to Word of God, to have no answer. Even so, the fanonical answer is almost as well-known as the riddle itself: "Poe wrote on both."
The Annotated Alice gives other speculative answers, including "Because they should be shut up" and "The notes they are noted for are not musical."
Silverlock implies another tack by having the Hatter ask "Why is an angleworm like a parallelogram?" and when challenged respond "I don't know as they're alike."
Originally the raven riddle had no answer, but Lewis Carroll got so many letters begging for an answer that in a later edition he offered a solution to the riddle. The answer is, "Because it can produce a few notes, tho they are very flat; and it is nevar put with the wrong end in front". The misspelling of the word nevar was intentional as it is raven backwards. But editors thought it was a typo and decided to "correct" it to never, which only served to make the answer nonscensical and to confuse fans for years on end.
Percy Jackson and the Olympians has several character facts that are fanon, the most prominent being that Annabeth can't swim. It does make sense within the canon considering she's a daughter of Athena, an therefore naturally born a rival of Poseidon meaning that her and water do not mix.
A lot of fans think Artemis has a kid she either hides or doesn't learn about for a long period of time. Artemis may be cool, but she's never been hinted at being a mother to anything but her hunters in spirit.
Fans believe that Nico has Roman Blood in him, because he was able to find Camp Jupiter without help.
Some think he may even have Egyptian blood, due to him looking like Anubis. Some even wrote him as his host.....
It's the general consensus amongst fans that ALL campers from a certain cabin will have matching hair and eyes.
Apollo demands the oracle is a 'virgin', so he can sleep with her. (Mostly in Rache/Nico fics).
The idea that Katie Gardner and Travis Stoll are dating is so widely spread that even the most knowledgable of fans get confused. Bear in mind they are mentioned interacting once throughout the whole series; and that was a throwaway comment about a prank Travis pulled. Yet in fanfiction they have almost as many stories as official couples Annabeth/Percy and Piper/Jason.
Most fans believe that Kurt, who appeared in all of one book and was never mentioned thereafter, was an undercover Vesper, despite there being no canon evidence supporting this.
It's generally taken for granted that Isabel Kabra nee Vesper-Hollingsworth and Arthur Trent ( revealed to be a Vesper) interacted with each other in their youth, with many fans going so far as to believe they were in a relationship prior to Arthur meeting Hope.
Quite a few fans are absolutely convinced that Amy's full name is Amy Hope Cahill.
Many The Hunger Games fanfiction stories give Peeta's brother (who is never named in the book or film) the name Rye. This is based off the idea that their father is a baker and because Peeta's name is vaguely related to bread (sounds similar to "pita"), that their other sons would have bread-related names too.