Acceptable Breaks From Canon

""griffon8 pointed out that I've been writing as though Shizune and Misha shared a double room rather than the singles every other student seems to use. Call it artistic license; it doesn't interfere too much with established canon (I think) and isn't even that critical to this story anyways.""

- Author's note, From Shizune's Perspective

While it's very difficult for a story with a big audience to ever reach a consensus, occasionally a single fact or aspect of the series is modified enough in Fanfic vs. the original story that its absence or modification becomes notable within the fandom. This may escalate to the point where fanfics do not bother to note the change in any tags or clarifications and that even sticklers for continuity are generally not bothered by it. This will also happen much more quickly in a tightly-knit fan community. Conversely, it occurs much slower if there's a big Fan Dumb community, unless they're being actively trolled.

Note that "mechanical" changes to make things work in a text medium (reworking a gag because describing a visual isn't funny) or changing an ending because the story covers that period ("soft" AUs) don't count. The change must be common enough to get fan recognition, deliberate rather than an outright mistake or forgetful Flanderization, but not actually overshadow the original version of the story (as Fanon can and does).

Common flavors include the following, and may occur separately or together:
 * Canon mixing: Borrowing a characterization or concept from another version of the story because they were more popular/interesting/notable even if the rest of the universe appears "normal". Frequent in series with many Alternate Continuities (and thus little sense of "true" canon), especially if those other versions aren't as necessarily so well known as others. In extreme cases, writers may create Fridge Brilliance to explain how even explicitly different versions are both valid interpretations simultaneously.
 * Continuity tweak: Avoiding or reworking something that just didn't fly in the audience's mind to make more sense or be less stupid. Again, can become a Fandom Theory with enough support.
 * Personality tweak: The toning down/up of a character's personality within "acceptable" bounds, said bounds usually based on... how people write the character in fanfics frequently. Popular with low-emotive characters who are otherwise difficult to write for or who have a popular line (count the usages of Big Damn Heroes in Firefly and then in Firefly Fics, for example).
 * Memetic Mashing: The writer incorporates the Memetic Mutation of a character into the work under the challenge of making it mesh believably with the established characterization. Difficult and prone to become very in-jokey if done badly, excellent Fandom Nods when done right.

Compare Fix Fic, where the story tends to focus entirely on addressing problems. Also compare Woolseyism.

Sorted by source media.

Anime and Manga
""Other than Yugi's long-lost cousin, Yami, mysteriously showing up a few months ago…""
 * Neon Genesis Evangelion: Mostly personality tweaking, like openly emotive Rei and more extroverted Shinji are the biggies; while never as popular as the show, some fans automatically assume the writer is using their manga counterparts instead. This might die out seeing as they've become official depictions in Rebuild of Evangelion.
 * Sailor Moon: Huge amounts of canon mixing, depending on what area of the world you lived in had in the way of Small Reference Pools and access to whatever source material was handy.
 * Zoicite: The harpy-like, melodramatic female incarnation from a single dub is widely more popular than the original, and tends to show up if the Dark Kingdom is featured.
 * Rei: Types A & B — Writers are much more likely to use her depiction in the original story (stoic, borderline Tsundere, "kinda gay") than the one in the show, even though the backstory for the favored depiction was not used there. In addition, due to the lines in the dub sounding very odd to those not familiar with Shinto mysticism, Rei was popularly depicted as having outright supernatural powers, something the original show only played with.
 * Makoto: Fanfic authors, especially male ones, downplay her whole obsession with her former senpai. Ironically, some fans disliked the Canadian dub for putting back references to a similar obsession with her femininity which had previously been avoided, because it made her "less cool" (incidentally making the official live action series invoke canon mixing).
 * Haruka: She might have whiter hair than expected, or be less combative or snobby (as seen in the manga) if the author is intending to make the Sailor Team more inclusive. Writers brave enough to use her "Amara" incarnation often rationalize her as simply a sort of "younger" prototype who is still in an ambiguous (but inevitable) relationship with her partner, discovering herself, play down her fashion choice, and compensate by making her a lot more sexually open.
 * Michiru: Usually hit pretty hard with the Cast Speciation hammer in regards to any traits she shares with Ami. She's also a musician OR a painter, but not both, under the idea it'd make her too idealized.
 * Dirty Pair: Most fanfic tends to be set in a continuity that's a varying mix of the classic anime and Adam Warren's "amerimanga" version, usually with the most memorable characters from both (Goulet, Nammo, Shasti or Cory Emerson, for instance). The psychic powers from the novels may also appear. Flash is either retconned as a Generation Xerox in the future, or alternately as just Outdated Outfits they wore when they were younger.
 * Tenchi Muyo!: The author's favorite characters or plot points from the many alternate Tenchi continuities get brought into the OVA universe. (This most frequently means Kiyone Makibi — and no, Noike doesn't count.)
 * Axis Powers Hetalia: Personality tweaking, in most cases. One example is how many fics take Japan's occasionally hinted-at Otaku and Covert Pervert traits in canon and play them up until he becomes a full-blown Yaoi Fanboy and/or conniving matchmaker for the main couple featured in the fic, most likely because he's one of those low-emotive characters difficult to write for.
 * If reading a Yu-Gi-Oh! fanfic, don't be surprised if all those ancient spirits have their own body, and don't expect it to be explained.
 * Lampshaded in this fic:


 * Also in, Decks Fall Everyone Dies, Yami has a separate body. There are hints that this will be explained, though.
 * Bleach has Ichigo and Rukia, two characters who are comfortable enough around each other despite dark pasts to be themselves - including the occasional friendly slap or kick for comedic relief, but on the whole a relationship ground in respect. Bleach fanfic on the other hand more often than not reduces them to foul mouth and ill tempered rivals who can barely go three lines of dialogue without insulting each other for no reason.

Comic Books

 * Teenage Mutant Ninja Turtles: Donatello is often written as insomniac and addicted to coffee, and as always falling asleep at his work bench or computer as he tries to solve some problem or challenge.
 * Fanfics based on the 1980s cartoon tend to downplay or outright ignore Raphael's characterization as the Motor Mouth Deadpan Snarker in that show and make him more like the violent hothead he is in all other continuities.

Film

 * In the Star Wars Forum Roleplay The Gungan Council, continuity is tweaked all around, yet not always to get rid of disliked event in Star Wars canon. Alderaan still exists, former Vongformed planets show little to no signs of ever being in such a state, and the Star Forge is still lurking in space.
 * Tron-universe fanfic will work with mixed canons (in the form of porting in aspects of the discredited sequel, Tron 2.0 (it helps that both timelines say Flynn and Tron vanished under mysterious circumstances) and continuity tweaks (often in the form of a "Flynn and/or Clu lives" scenario).

Literature

 * The Peter Pan fic The Lost Girl tweaks personalities at some points.

Live-Action TV

 * In Buffy the Vampire Slayer fics, especially during Season 6 fics and Post-Chosen Fics, personality tweaking is loved. A common way to get rid of Kennedy in Post-"Chosen" fics is for her to be a cheating bitch, an abusive bitch or a cheating, abusive bitch. A common thing to do with Buffy is take her Season 6 angst and make her an Emotionless Girl faking emotion.
 * The death of  at the beginning of Season Seven of Supernatural has led to so much continuity tweaking it's kind of mind-boggling.

Video Games

 * Touhou: Expected, since Touhou runs on Alternate Character Interpretation and Memetic Mutation
 * In the Katawa Shoujo fanfic "From Shizune's Perspective," which was written prior to the full game's release, Shizune and Misha, as well as Rin and Emi share rooms. In the full game, all the rooms appear to be single ones, although Hisao never goes to Shizune or Misha's rooms. Shizune and Misha sharing a room becomes relevant when Shizune, looking for Misha and Hisao, enters her room

Western Animation

 * Teen Titans fics in terms of which Robin is in the series. Some have Tim Drake and others have Dick Grayson, which may lead to a change in personality (depending on how familiar the writer is with the comics) and sometimes even a change in name. (Nobody wants to have a character named Dick). It is hinted in the show, and confirmed in the tie-in comic, to actually be.
 * All over the place with Adventures of the Galaxy Rangers, usually cranking up the Darker and Edgier tone, playing some of the more unsettling aspects completely straight, or offering some Deconstruction and Reconstruction behind the characters and universe.