The one that leaves you going...W...T...F? So many examples I can't think of one right in particular-oh, wait. Yah. I don't remember what game it even was, but there was this one that I'd walk into a room and...die. Seriously. No bad guys, no bullets, no warning. Just...die. I never did find out what the heck that was. The save point was a good fifteen minutes of gameplay away, so it wasn't worth figuring it out to me.
Oh, and platformers in general, when you THINK you can make the jump,and it LOOKS like you could make the jump, but your little dude's legs are evidentally ONE INCH too short and you fall into a pit of insta-death. Splut.