One that was burned into my mind as a child was some Biblical epic where a man was executed by being pulled between two horses, and as he separated at the waist, it showed him being dragged off with the tattered remnants of his clothes dragged over the furrows in a field.
When I think of how much I've forgotten over the years, that image is absolutely and indelibly branded on my psyche.
A more recent one, and this shows how bad my memory is, because I can't remember what it's from (Band of Brothers, Stalingrad, something like that..), but it showed a fight in a house between two soldiers and it descended into a punch up (not sure if they ran out of ammo), and at the end of the fight, the bigger soldier lay on top of the defeated one and told him quietly to shush, and he pushed his knife into him. I think he may have put his quietning finger on his mouth as you would a child.
I'd be grateful if anyone remembers the film and the scene, as I'd like to watch it again.