RoboCop
When I was a kid I heard Robocop way before I ever saw it. There was no way my family would have ever watched something like this, but when we went round to my cousin's house they had it on the TV, and thinking of the timing they must have bought it as soon as it came out on VHS, because I must only have been about 8 or 9.
Anyway they were happy enough to let me watch a bit of it when it just seemed to be a movie about a silly robot policeman, but when that truck full of acid burst open and covered Paul McCrane that's when I was told in no uncertain terms to stare at the ground until it was over. I remember my cousin describing what was happening though, and that - mixed with the sound from the TV - probably impacted me far more than if I'd just watched it.
It makes me wonder, what will I do when my son is old enough to watch movies that are generally too old for him? I can't imagine exposing him to anything like this anytime soon (he's three), but at what age do I want to start scarring him for life?!