Um, the curbing bit in American History X makes me want to puke.
I couldn't watch that scene in its entirety, it was too fucking disturbing - more so because it wasn't some hack 'em up horror, but something that has more than likely actually happened.
Like Joe said,
Hard Candy had a scene that had to be every man's worst nightmare.
The Cook, the Thief, His Wife and Her Lover - pretty much everything in this pretentious crap is designed to disgust and nauseate the viewer, from castration to cannibalism. Pretension masquerading as art. The whole film is fucked up and disturbing.
The scene in
Se7en where the guy is talking about how he fucked the girl to death with a knife-strap on. Thinking about that
still disturbs me to no end.