There's this scripted event in
BioShock, at Medical Pavilion. There's this dentists office where you have to go in through a window and unlock the door from the inside. There's a turret behind the counter.
You go into the dentist's working room (whatever it's called where he actually does the stuff) and the room whites out in mist. After hearing footsteps, the mist clears and now there's a corpse in the dentists chair. Over to the right is a tonic. Pick it up, set it. This is where the real fear came in for me.
After setting the tonic, I started walking backwards. Couldn't move. My first thought was "Dang it, stupid game." I turned around, and standing right against me, weapon drawn, is this dentist. I freaked out and beat him to death with my wrench, scared as hell.
I heard he only attacks back if you attack, but you can't move until somebody attacks anyway...
* MegaScientifical shivers *