I've got some "oh...shit"-moments myself:
In Half-Life 2, in the start, when you meet the first zombies and one of them pops up from a radioactive pond, I just thought "What the hell is THAT?" Because it was the first time I've ever played a Half-Life-game.
Another time in Half-Life 2: When you encounter the first "carrier"-zombie in the game, I went; "oooh, shit, I HATE poison headcrabs, and he's got like 30 of them on him!"
Half-Life 2: One word: Ravenholm.
Half-Life 2: Episode 1: When you're in a room and an Advisor pops up and sort of uses mindwaves, it's freaking unexpected the first time!
Half-Life 2: Episode 1: The Elevator Battle (
http://www.youtube.com/watch?v=JMK0HtKUHCQ for those who don't know). I just hate it, although it is an epic battle.
Half-Life 2: Episode 2: The ambush, epic battle, but an "oh shi.."-moment, also the last battle, when there's like 4 striders approaching the rocket, epic battle. COnclusion: The HL2 games are full of epic battles.
God of War: In the start, I was playing for the first time on HARD, and when I've finally defeated the first Hydra-head, I encountered another one a minute later, I just though: "Oh fuck, not again?" And later, when I encountered the REALLY big Hydra-head, I went all

.
We've also got the part in the desert, singing Sirens, you just see them in the horizon through the thick sand, and suddenly, they're right behind you, and of course, you've got the volume at max to hear them better. I went "Oh shi#¤%!".
God of War: The part in Pandora's Temple, when you've reached the second circle or so, when there comes a BIG SPIKY concrete-tube rolling at you.
Call of Duty 4: When you're in Russia or something, and you see the nuclear rockets launch right in front of you, and everything goes into slow motion.
Also the part where the americans get hit by the nuclear bomb's shockwave and that man flies out.
I think that was all
