Pro Ana sites. I ran into one of those long ago, and the "helpful tips" these people gave each other, along with the justification and encouragement, still sticks with me.
A friend of mine has a brother who is somewhat deranged, and he once posted an old pic from a catalog of terrible accidents that had a child's arm stuck up to his shoulder in a meat grinder. Yes, there was meat coming out of the grinder, and the child was obviously passed out. You know that thing where you get all trembly and weak because you can imagine the pain of someone else? Even a flicker of that image on the edge of my memory gives me that sensation. I'd delete it from my brain if I could.
Oh, and I wish I never saw the movie Cropsey. It started out interesting and creepy and slowly devolved into something depressing and unnerving and terrible. For those who haven't seen, the documentary creators set out to find the origins of a Staten Island urban legend they grew up with about a boogyman creature that steals and murders children. Slowly they uncover that this legend probably popped up from a set of real crimes involving a man who kidnapped, raped and killed mentally challenged kids on Staten Island. He also squatted in an old, abandoned asylum (which has a pretty nasty history of its own, and now looks like something straight out of a horror flick). Eventually they contact the murderer, who is now doing time, and he's all sorts of crazy. It was terribly sad and awful and there seemed to be no point to the story besides finding out that sometimes urban legends are created from real tragedies.