This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Funny Games
Terrifying not because of any of the usual horrific creatures or supernatural occurrences, but because it's just people, there's never any question of that, and they do it all with friendly, reasonable smiles. You'll be looking over your shoulder for days. "Can we borrow some eggs?"
The way one of the killers rewound the movie after the mother kills one of the pair; completely changing the scene, can be very disturbing.
Pretty much anything related to thrill killing is some kind of Nightmare Fuel. It only makes it worse for people who are fascinated with true crime. It wassobad that this is virtually the only scene about which you could say had any legitimate drama in it.
The US version was off the hook because of the presence of Michael Pitt. There's just something about him that lends itself perfectly to sociopathic roles.
Even just the sight of the DVD cover for the American version can make one physically flinch away after seeing the original Austrian film.