Canon Defilement

Fan Fiction 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) over what they see as Canon Defilement.

Also worthy of note is that many cases of traditional canon defilement actually are canon, but in those cases, it's always when a Promoted Fanboy gets to 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 Seasonal Rot/Creator Breakdown.

An incomplete list of things that are generally known to cause this:
 * Canon Character Derailment in general (often referred to as Out of Character or OOC).
 * Radical Backstory alterations, whether it be as part of the Possession Sue process or to set them up as a heel so they can Die for Our Ship.
 * Changing their personality in ways that majorly conflict with their established mannerisms (again, for either Possession Sue or Die for Our Ship).
 * (Regularly) Disregarding their capabilities or giving completely new and unlikely ones in order to facilitate the plot.
 * Related to the Character Derailment is the creation of Mary Sue in any of its incarnations. In order for Mary Sue to exist, the universe has to bend around him/her and the first thing to happen is for the all the characters to become anchored to him/her (whether it be out of reverence or contempt). After that, the usual Mary Sue story can play out, basically turning the canon into little more than window dressing for the Wish Fulfillment of the author.
 * Major Genre Shifts.
 * Disregarding established 'Verse laws in order to facilitate the plot without offering any satisfying explanation as to why.
 * Literal Rape Tropes 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 Fics and especially the highly-contested High School AU. 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 Fics. This is especially true in relation to fandoms that skew old, such as Star Trek (or at least Trek before 2009), where there are few High School AU stories because most writers are in their thirties or older.
 * Sailor Earth, in general, is often viewed as this.
 * The breaking up of Official Couples for any reason, whether paving the way for Fan Preferred Couples, Crack Pairings, or Relationship Sues.
 * The great Slash Fic debate, along with the Lemon debate. The former has heavy ties with the Character Derailment argument.
 * Alternate Character Interpretation 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."
 * Elsewhere Fic can also be considered Canon Defilementagain, be careful with throwing this around.
 * Fanfics built around "What If?" scenarios (eg, assuming that the characters decided to Take a Third Option) are, once again, sometimes viewed as Canon Defilement.

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 Did Not Do the Research or perhaps just didn't care. Often, though, it's an attempt to make it Better Than Canon. Sometimes it's played for humor, but with something like this it can be hard to tell...

Also known as Canon Rape.