One of the things that drives me nuts in fanfiction is the phenomenon of making canon characters out of character, something we like to shorten to OoC. Why is it that some fanfic writers think that it is actually all right to turn characters OoC? Why is it that writers think that supplying the reader with a warning that characters are going to be OoC is all right too? Truth of the matter, it isn't.
Let's start by discussing, what does it mean by being OoC. To be out of character, that means that the character is doing something that they normally wouldn't do under these circumstances. There is no explanation as to why these characters have suddenly changed from the way that they normally are. It is almost as if they are a clone of the character, or as I like to put it, an OC (original character) disguised as an canon character.
There are circumstances where a character is not OoC though, when some people might. I can draw from examples of where I've been accused of writing characters OoC. One was with Aang from Avatar: The Last Airbender, when I focused on his depressed side for one of my fanfics, Sister Avatar, currently being rewritten. The other was when I had Matsumoto Rangiku from Bleach mentally snap, and do something she normally wouldn't do, only to regret it later (Glacialus Sub Bui). But many of the times I see it, it isn't explained away.
I read fanfiction, so that I can read about the characters I love so much, because I can't get enough of them from the canon source. I want expansions of their characters, not something that smashes them to pieces. I also feel, if you have to change the characters to work for your story, instead of changing your story to work for your characters, you're doing something wrong.
But what is even more disturbing, at least from my perspective, are the writers who make the characters OoC because they are in truth writing themselves, and their experiences, even though those experiences are not ones that would fit with the main characters experiences. Not being able to separate oneself from characters is a bad thing, for many reasons.