Condemned : A moment in a clothing shop filled with mannequins, and you notice one moving, and saying to yourself, am I going insane? and then you turn arround, and get one attacking you.

Painkiller : Every Boss fight. KICKASS.

The Nukes in both COD4 and Fear.

Xhumed:

The ambush mission in Mechwarriors: Mercenarys- i knew it was going to happen, but it still elicited a "crap!" when it happened

Mechwarrior 2

Final Clan Wolf Mission.
Getting a radar contact that turns out to be a Warhawk. Then getting a second blip, that turns out to be another Warhawk.

in tales of symphonia going through remeil with 2 hp left on llyold and everyone else dead then fighting kratos
having the brightness too low on the tv in gears of war and hearing............"boom"
and in metal arms glitch in the system when they put you in the coliseum to fight general corrosive i almost peed my pants

lol When i got eaten by krill in gears of war for the 1st time. i was liyk wat da fuck man

Having several teams of Exceptional Soldiers on my island in Evil Genius, mowing down all my minions and having rendered my Henchmen unconscious, when I hear the little music sting that accompanies the arrival of that bastard John Steele was an "oh fuck" moment. Followed by several more when 2 more Super Agents arrived. 5 minutes later the final one when they all enter my base at once and proceed to kill all my remaining minions and destroy my base, resulting in having to reload a save from 45 minutes earlier to avoid the same thing happening.

Bioshock, near the start when you walk into that dental office. You see the guy's shadow leaning over the table, then the lights flicker and he's gone. You go round and he's not there. Then you're on your way back and he jumps out of a locker next to you when it's really dark. Needless to say he got a few more whacks from my wrench than was entirely necessary.

Guitar Hero 3 on Hard Career Mode. I finally finished Raining Blood after nearly pulling my hair out only to be confronted with the Lou Boss Battle. Yeah, colourful words were said (though thankfully after many expletives, I managed to get through it tonight).

Well, let's see. I have plenty...

Gears of War: Room full of radioactive wretches.
Majora's Mask: Ugh... seeing the boss of the water temple for the first time.
Ocarina of Time: When Link gets totally pwnd by the giant shadow in Kakariko Village.
Dead Rising: seeing a wave of 5 special forces soldiers headed towards you.
Condemned: That one guy in the locker who's dead. *SPOILER* OR IS HE!?!? *END SPOILER*
Guitar Hero 3: When you realize you just beat TTFAF on expert, and wasted your life.
Oblivion: When you walk into the city overrun by daedra, and BAM! There's Mehrunes Dagon.
Splinter Cell: When you start sneaking up on a guy, and the lights suddenly turn on.
F.E.A.R.: Every freaking Alma moment! I HATE THAT BITCH!!!

This is one i did to someone else while playing online o CoD4:
I was playing as a sniper and the match had just started, when you play as a sniper you get a really usefull ghilly suit, it works. I had laid down in the grass waiting for a kill when this enemy runs up to me and crouches a foot in front of my face. He had come around and was in a great flanking position and was about to spray my comrades with an M60, he hadnt seen me. I wated about half a second then jumped up and stuck my knife in his gut. I heard the guy yell OH SH*T in to his mic. Me and my friend laughed about that kill for hours

TheNecroswanson:

TankaX:
Hey,

In Deadrising when i'm in the North Plaza and after i plow through all the zombies to the gun shop and then as i enter the crazy guy Cleaus keeps shouting at me and this other dude and the guy walks foward to the crazy and he gets shot in the chest and goes flying out the doors right into zombies...

Also in Champions: Return to Arms after i beat the shit out of four evil knights and the zombie a gaint door opens reavealing a gaint fucked up five arm thing and it's like 20x bigger then me!(When i finally beat it a mother fucking super gaint warlordcomes out and is 250x bigger then the thing and i thought i had to battle him... but didn't).

From, your friendly gamer TankaX

Yeah I rather diliked Innorouk. Though he was fun to pummel.

Innoruk shall CRUSH YOU...

I <3 EverQuest moreee

Confronting the FIRST boss in Shadow of the Colossus

Edit: Woops, didn't mean double post. Forgot I already posted >.<

a big oh shit moment for me, was when i was playing Rainbow six vegas. i came sprinting around the corner to try to get to the objective and five people on the other team were heading and looking my way. i got killed before i could even turn around

another was when i was playing gears of war. my team was looking for the last 2 guys. we were staying together to be safe, and two snipers popped out from cover and shot both of my team mates in the head at the same time.

doing 99 damage in 1 hit with a deagle (i always check console for damage done :P)
i do this more often than is natural

There's a point in BioShock where you go down into this basement room that has knee-high water and a bunch of mannequins and a powerup on the other side... the first time i played the game, i just ran across the water and picked up the plasmid and i think there was a safe or something... then when I turned around one or two of the mannequins had turned into creatures and it made me scream... but I killed them and turned back around and then suddenly more had transformed and it made me scream again... it was a bad moment cause usually I don't get that freaked by a game...

Also going with Bioshock:

Any encounter with SA-X in Metroid fusion.

AlphaWolf13:
Confronting the FIRST boss in Shadow of the Colossus

Or the second...or the third...or the fourth (etc,etc)

The first time I tried to take out a Splicer in Bioshock with that flying bees plasmid and didn't realize that the bees will indiscriminately attack anything in the vicinity. To which I managed to piss off a Big Daddy when I was already low on health and he stomped on me like a football player coming off a fresh injection of rage inducing steroids.

then there are the Fatal Frame games that pretty much had you going "Oh shit!" throughout.

Play the secret level "GO 2 IT" in Final Doom: Plutonia Experiment.
...Or look it up on youtube.

Realising I had to fight 25 Metal Gear RAYS with no Rations.

Playing Goldeneye Multiplayer with proximity mines... pretty much a constant "oh shit" moment, mostly down to forgetting where we'd put them. Also that freaking dinosaur you have to fight on Champions of Norath, until i worked out i could hide behind the pillars. When i worked that doozy out, it was easier than the preceding clusterfuck gladiator fights.

The ending part of No More Heroes, the last two fights.

In Final Fantasy X2, when you play it for five minutes and realized you just waisted 50 bucks

hmmm in medal of honor: spear head, you have to capture a 88 gun, so i do that, and then after you that a tank comes flyin through the wall, and runs over ur teammate i was like OMFG YOU BASTARDS WHATS GOIN ON!!! lol i was stunned for about a day

the first time playin any metal slug game every boss fight was like man shiiittttt

mariokart 64 when everyone has one balloon and a red shell

Last Boss in Crysis. "Oh... Shit, I have to ficht that thing? Sweet Jesus."

The battle with the Widowmaker inside the Chimeran hub.

Laughing Octopus. Her hiding, her strangling, her bombs, her laughter. It was all terrifying.

EVE Online - Warping to a Stargate in Lo-sec, encountering the biggest ammount of pirates I've ever seen... I thought "Oh sheit!" And then i was in my pod...

DEFCON - Seeing the entire enemy continent explode in "Nuclear Launch Detected"-icons.

Rainbow Six: Vegas 2 - Me and two mates where standing by a door, unaware that the enemy was on the other side of that door... Just as we are about to enter, the door explodes, throwing our corpses several meters back, those bastards had placed a breach charge D:

Orbiter - During a trip to the moon, i was messing about with some of the spacecrafts system, i look up and notice the moon filling up my windscreen, rapidly approaching...

A whole bunch of Resistance

every time followed by... oh thats right, its resistance, and then i would absent mindedly blow away whatever wonderfully rendered soap-bubble enemy they had conjured this time.

I got one. Ninja Gaiden: Black the first time you fight Duku and you can't hurt him. You try and win that one then.

LoZ: OoT. The Water Temple.

Laregly because I'd heard people use the phrase "The Water Temple" to describe something that was impossible, and I just rolled with it and never learnt what it meant. A few months ago, I got out my N64 and played on Ocarina of Time. I progressed through the game, and I felt the colour drain from my pasty face when I saw "The Water Temple" come up as where I had to go next.

The first time playing The Cradel in Thief: Deadly Shadows. I walked next to a puppet and it got up. It scared the living crap out of me and I ran screaming into my kitchen.

Good times, gooood times...

When you reach the roof of the city hall in Half Life 2, then have to fight 2 striders and a gun ship.

First time I saw the striders the first words form my mouth were literaly "oh Shit"

The hallway dog in Resident Evil. No single event has yet come close to this level of mind-buggering horror, though the first 30 mins of Silent Hill and Adam (the psycho clown) in Dead Rising were close.