Yami no Hitokiri
It is very annoying. I read that error countless in fanfics and twitch every time I come upon it. I'm such a grammarian that I find myself unable to enjoy the fic as much. It just glares out from the webpage and mocks me by saying, "You can't do a damn thing about it." I seriously need to chill out.
sad I know all too well of what you speak. I constantly read fanfiction, and find that no matter how well-written the story is, the grammar errors always, always, ALWAYS seem to stare you in the eye. Then they proceeed to make you loose sleep and start to hate the fanfic.