Half-Life - my first encounter with a barnacle. I'm standing there, listening to what that doc has to tell me when suddenly the ceiling would come crashing down and through all the rumble I see a barnacle lashing down and pulling that doc up. After about five seconds with a "WTF?" written on my face, and about the time blood and bones would start raining down, I would just quicksave & alt+f4.
Bioshock - the morgue scene at a pretty early point of the game, when you're walking slowly through water to where you're seeing a shadow from some doctor doing... stuff... being reflected on the wall, shortly before you get around the corner to wtfpwn his ass the lights would go out. My first reaction, aside from getting THE HELL OUT OF THE WATER (Which resulted in me running against the wall like an idiot until the lights would go back on.), was to switch from revolver to shotgun. You get around the corner and the only thing you see (Aside from his "experiment" he was working on.) would be one of those giant locker doors being shut down on you. I was freaking scared on my way back, but when he finally leaped out of one of the doors on the other side I'd put some shotgun shells strangely accurate between his eyes - that doesn't help the fact that I spent another three to four shots spreading barrage through the room in a "FUCK THIS SHIT!"-moment, though.
Here's a video from that scene.