There's a scene where a character is being killed behind closed doors. We just hear really, really awful noises as a large amount of blood begins flowing out from under the door.
The Engineer's first appearance could also count.
The Torso Cenobite, which would be considered a copy of Chatterer... except it's just the upper body of one of the Chatterer's experiments, made to look in its master's image.
The ending: Forced to relive your actions and watch everyone you love die for all eternity.
