{{Fanfiction}} communities, like any other fandom, have their own set of standards. While technically, all fanfiction is free to just do whatever it wants (hell, all fiction in general is, too), there are a number of topics and tropes that cause revulsion amongst these communities. The revulsion can range from forgivable (most beginning writers are given a free pass for this so long as they aren't particularly mean-spirited) to sending whole communities up in arms (which will sometimes result in being banned as a particularly light sentence).

Also worthy of note is that many cases of traditional canon defilement actually are canon, but in those cases, it's always when a PromotedFanboy gets to [[RunningTheAsylum Run The Asylum]] and just abuses editorial fiat. It takes quite a lot more for them to really get this term thrown at them, but it generally tends to sting a lot more, since their influence will (usually) affect future stories as well. Please note that it isn't Canon Defilement if the original creative crew is responsible; it's just good old SeasonalRot[=/=]CreatorBreakdown.

An incomplete list of things that are generally known to cause this:
* Canon CharacterDerailment in general (often referred to as OutOfCharacter or OOC).
** Radical {{backstory}} [[{{Retcon}} alterations]], whether it be as part of the PossessionSue process or to [[RonTheDeathEater set them up]] as a [[AssholeVictim heel]] so they can DieForOurShip.
** Changing their personality in ways that majorly conflict with their established mannerisms (again, for either PossessionSue or DieForOurShip).
** (Regularly) Disregarding their capabilities or giving completely new and unlikely ones in order to facilitate the plot.
* Related to the CharacterDerailment is the creation of MarySue in any of its incarnations. In order for MarySue to exist, the universe has to bend around him/her and the first thing to happen is for all the characters to become anchored to him/her (whether it be out of reverence or contempt). After that, the usual MarySue story can play out, basically turning the canon into little more than window dressing for the WishFulfillment of the author.
* Major {{Genre Shift}}s.
* Disregarding established [[TheVerse 'Verse]] [[MagicAIsMagicA laws]] in order to facilitate the plot without offering any satisfying explanation as to why.
* Literal [[SexualHarassmentAndRapeTropes Rape]] [[RapeIsASpecialKindOfEvil in any form tends to throw FanFic communities up in arms]] (excluding perhaps, instances of rape that took place in the actual canon).

There are other things that a good portion of fanbases add to this list, but that many other people really like (or at least think it adds to the story) and are willing to defend:
* This is a common accusation leveled at {{Alternate Universe Fic}}s and especially the highly-contested HighSchoolAU. The latter especially tends towards being little more than the writer playing out their personal fantasies while trying to appeal to readers with a vestigial connection to a canon.
** Although it should be pointed out that in canons where time travel, alternate universes, and the like are canonical, there is little to no controversy over {{Alternate Universe Fic}}s. This is especially true in relation to fandoms that skew old, such as ''Franchise/StarTrek'' (or at least ''Trek'' [[Film/StarTrek before 2009]]), where there are few HighSchoolAU stories because most writers are in their thirties or older.
* SailorEarth, in general, is often viewed as this.
* The breaking up of {{Official Couple}}s for any reason, whether paving the way for {{Fan Preferred Couple}}s, {{Crack Pairing}}s, or {{Relationship Sue}}s.
* The great SlashFic debate, along with the {{Lemon}} debate. The former has heavy ties with the CharacterDerailment argument.
* AlternateCharacterInterpretation is sometimes even viewed as Canon Defilement. Typically, whenever you lodge a complaint at a fanfic for interpreting the characters a ''different way'', expect to typically be ignored or seen as nothing other than a {{Jerkass}} or {{Troll}}. Sometimes, people often cry Canon Defilement at this merely to say "My interpretation for ''this'' character is better than ''your'' interpretation."
* ElsewhereFic can also be considered Canon Defilement, again, be careful with throwing this around.
* Fanfics built around "WhatIf" scenarios (eg, assuming that the characters decided to TakeAThirdOption) are, once again, sometimes viewed as Canon Defilement.
* Some people view SelfFanservice as Canon Defilement. This largely [[DistractedByTheSexy depends on the person.]]

It is important to note that Canon Defilement ''does'' have a subjective side to it. Sometimes, one person's Canon Defilement is another person's fanfiction idea - and criticisms about it being Canon Defilement sometimes get responded to with a big ignore.

This generally comes about because the author either didn't know better or perhaps [[TheyJustDidntCare just didn't care]]. Often, though, it's an attempt to make it BetterThanCanon. Sometimes it's played for humor, but with something like this [[PoesLaw it can be hard to tell]]...

''[[ExampleSectionectomy No Examples Please!]]''
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