Definately two moments that got me in Bioshock... when you're in the doctor's labs area.
*Maybe spoilers for people who don't like to know what's about to scare them witless*
First one is a splicer lying on a trolley, pretty dead. You go to a cabinet thing next to it and rummage around (as you do), turn round, and it's lifted its head up, turned to face you and is pretty much grinning at you. I smacked that damn thing with the wrench for about five minutes for that...
The second one is the same region of Rapture, except when you do the rummaging all that fog happens, turn around and there's one right fucking behind you, just... standing there. I was more determined to kill every single thing I saw after those two.