Every single fanfiction where they all of sudden turn all the characters into sex-machines... I can't say I have anything against sex, but you just don't turn great characters into sex-machines. You just don't do that.
Oh, god... the horror of it. It's was removed from fanfiction.net a loooong time ago, but if you google it it should come up. Just don't say I didn't warn you!
I've somehow actually managed to avoid the disturbing/too strange ones (thank Helena). I heard there's was a Harry Potter one about Hedwig and I think it was Harry, though. I really don't even want to know what happens in that one...unfortunately that's one where you can't stop your imagination from trying to figure it out because it's just that disturbing and weird... >.<