Fighting an epic 6v6 confrontation in the Chromehounds online campaign mode then realizing that the entire battlefield is literally aflame. One of my squad mates was armed to the teeth with incinerary-round-loaded howitzers.

After every last one of us blew our radiators, incinerary rounds were forever banned from our squad.

Gotta love that classic moment in the first Resident Evil when you walk down that hall and the dogs come right through the windows. Good stuff. And while RE 3 didn't get much right it did get the whole Nemesis chasing you through rooms thing down pretty well.

Semi-major spoiler moment:

Trying to talk to Klavier Gavin near the end of Apollo Justice and seeing FIVE Psyche-Locks descending in front of you. And then again seconds later when you realize they're black, and even our man Phoenix can't unlock them.

The T-Rex in Tomb Raider was a particular shock. Back then I didn't research my games as much so was completely unprepared. I thought the raptors were freaky enough but when the screen started shaking and the T-Rex lumbered from behind that huge mountain, looking right at me as he roared, my heart lept (I was 11, bear in mind). Such a pity that the otherwise enjoyable Tomb Raider Anniversary didn't capture that moment with half the energy of the original.
A few others include the Zombie Dogs breaking through the window in the original Resident Evil, being chased by a Big Daddy for a whole 2 minutes having pissed all my ammo up the wall in Bioshock, first encountering those shit-scary nurses in System Shock 2, and a highly different kind of "Oh shit" moment after I bought the original Blinx game for the Xbox and realised it was the worst purchase I'd made in a long time.

finding out that the tony hawks franchise has well and truly been flushed. After a period of time playing project 8, i just yelled at the TV "oh, shit." add "piece of" into that statement and you'll get an idea of what i meant to say. plus turning on spyro: enter the dragon for the first time and seeing what spyro now looks like made me shout "oh... shit." then i fainted. woke up after five years. turned off the game. sulked, and because i was now a teen, swore a lot more.

My friends a freak at Halo 2 so one day we were at his house and he said okay guys 3 vs. 1 let's see if you can take me. Mind you only one of us actually owned Halo 2 or even owned a xBox at the time. So we kindly accepted thinking that he was just trying to piss us off and act better then us.

....

We were wrong. He killed us every which way and he beat us sometihng like 50 to -1. We may just suck but the funniest thing that happened....

We started sticking together thinking he kills one of us then we kill him. He somehow comes from the cieling with one fo those wierd Halo 2 glitches and fucks us all up. Ultimate oh shit moment for me right there.

*** Potential Spoiler ***

In Vampire - The Masquerade: Bloodlines, quite early on you're sent to a haunted hotel. There's a moment on the first floor landing where you're walking around and there's suddenly the ringing of a bike bell along with a glimpse of something else (ripped straight from the Exorcist)

Despite it's obvious 'inspiration', it still inspires a moment of pure WTF!? first time around (unless you've just read this)

Running into a room in COD4 multiplayer and seeing the enemy with an RPG. Easy kill, I thought then heard the distinctive WOOSH of the RPG launching. Killed us both.

Just about every moment in SWAT 4. Especially being shot in the face after peering behind a door.

I'd have to say the whole ending to MGS 3. Having the satisfaction of killing that defector, commie bitch, the Boss only to find out that she was in fact working for America and sacrificed herself for the country. And finding out that that cocktease EVA was actually a spy for Mao, I was simply stunned.
Also got to throw out Sonic for the dreamcast when you were being chased by the whale, simply stunning for its time and that game was awesome.
Finally, every single second playing POSTAL 2. Whether it be pissing on someone then throwing matches to light them on fire, using cats as silencers, throwing anthrax infected pig heads, smacking someones head off with a shovel, etc... This game certainly was 10 times worse than anything Manhunt 2 had to offer and it only got an M rating.

Ok, I have three:
-Your car breaking down in the maintainence tunnel in Dead Rising. Oh shit! Zombies everywhere!
-Dead Rising again. Finally breaking out only to find you have to fight a tank.
-Bioshock. Shooting bee's at people only to find that aorund the corner was lurking a Big Daddy who is now covered in your bees and none to happy about it.

COD4:Running through a hallway and randomly hearing "click"
Portal: Descending into the fire pit (probably already said)
Red alert 2: When you see 3 kirov ariships shaded in black...

bioshock - when you walking in that water filled hallway watching the silouhette, and then the lights go out. or a little big later when you look at that cabinet and then turn around (you know which one). first time you see a big daddy go to work

any game - low on health/no ammo/very low level/crap for items, and you walk innocently into a room that is then sealed off while huge towering boss reveals himself.....and then realizing that you haven't saved hours.

CoD4 - the end of Act I as soon as you rescue that helicopter chick. i've never had a more stunning moment in my video game life. not even aeris comes close

Odin, Bahamut, and Alexander cutscenes in FF9. and OZMA

FIRST TIME I SAW CRIMSON ZOMBIES IN RE1 FOR GAMECUBE

There's a mod, called Zombie Panic:Source. Playing as a survivor and losing sight of the others, hearing a zombie behind you. You spin to shoot him, BUT THERE ARE 19 OF THEM. Add to this hearing the click, click, click of an empty pistol, and you've got a change of pants.

Resident Evil: Walking past the window in the first part and having the zombie dogs smash through it...

FF7: Emerald & Ruby Weapon

(spoiler?) Megaman ZX Advent: At the end of the game , on the ouroborus(sp?), Having to do all those boss battles, getting to the the final boss and having no sub-tanks.

Resident Evil 4: Finding out I have to drag Ashley around.

Sorry if these have already been posted.

skyfire84:
bioshock a little big later when you look at that cabinet and then turn around (you know which one). first time you see a big daddy go to work

Yeah man, scared me shitless first time. "OH SHIT! *BOOM*" Good thing he doesn't move at first, eh?

This may sound lame, but Crysis held several for me - facing the six aliens onboard the spacecraft, and then seeing the giant boss rise out of the ocean. First time at least.

Lets see...

The most recent one would have to be in Call of Duty 4, in the last few moments on the bridge, especially when that guy starts coming towards you as the others in the squad are killed.

Then there was Half-Life 2:Ep2, that whole section with the antlions, when your trying to protect alyx as she's being healed.

In Halo, the first experience with the flood (Theres too many of them!)

A few times in Resi4, when I'd look to the side and see someone about to get me, usually the chainsaw dudes.

And going back to the genesis days...
Sonic 2, when I realised I had to fight 2 bosses in a row without any rings.
And Sonic 3, when I first saw the final boss, because of how difficult it looked to hit him.

*SPOILER*

When I was playing Portal, wondering how the cake would look like at the end of level 19, and then turning the corner on the moving platform to see a sea of fire with the words "4000 degrees Kelvin" uttered by GLADoS.
Took me a few turns to figure that one out.

In Supreme Commander, when kicking an enemies army all the way back to his base, on to hear the dreaded "Strat- str- strateg- strategic launch detected" and being completely powerless to do anything about the constant beeping and the four massive explosions as your base is wiped from the face of the planet.

In COD4, any time I hear the click of a claymore.

The final colosus, in Shadow of the Colossus.

In any Total War game, any time I realise I'm now defending a city with 200 spear militia, against a fullblown army.

In portal, On the incinerator map, I found myself thinking:
"Come on, Valve wouldn't let it end like this right? They made a way to escape... right? Come on deus ex machina... Damnit, show me the exit allready! Aaargh!"

The T-rex in the original Tomb Raider. Heck, I *know* it's coming now, and it still gets to me.

The Ogre Tower in Onimusha Genma. One level there's TWO red minotaur demons.

Alcari:

In portal, On the incinerator map, I found myself thinking:
"Come on, Valve wouldn't let it end like this right? They made a way to escape... right? Come on deus ex machina... Damnit, show me the exit allready! Aaargh!"

If you weren't sufficiently absorbed by the psychology of the Portal Gun to immediately realise what to do by that point, you don't deserve any cake.

In COD4, when you're a sniper with the badass SAS dude and you dive under a tank to hide or when you're just supposed to lie still while a whole battalion drives by...holy shit

Front Mission 3, when the bomb goes off in the city, pure devastation

FF7...I didn't know when it happened, and...oh my god...I could have sold that Princess Guard..I WENT TO THE GOLD SAUCER WITH HER! And then...at the end, in the lifestream...? Wow!

True crime streets of LA, when your on a mission with a swimming pool in the middle...loads of guys and your cover diminishes fast

GloatingSwine:

Alcari:

In portal, On the incinerator map, I found myself thinking:
"Come on, Valve wouldn't let it end like this right? They made a way to escape... right? Come on deus ex machina... Damnit, show me the exit allready! Aaargh!"

If you weren't sufficiently absorbed by the psychology of the Portal Gun to immediately realise what to do by that point, you don't deserve any cake.

What cake...?
THE CAKE IS A LIE
THE CAKE IS A LIE
THE CAKE IS A LIE
THE CAKE IS A LIE

Sentient Muffin:
Fighting an epic 6v6 confrontation in the Chromehounds online campaign mode then realizing that the entire battlefield is literally aflame. One of my squad mates was armed to the teeth with incinerary-round-loaded howitzers.

After every last one of us blew our radiators, incinerary rounds were forever banned from our squad.

The thing is, if you have decent cooling, Incendiaries are actually shit, they won't heat you up fast enough to damage you before you can get out of the area if your cooling is high enough, and they have to be relatively accurately aimed. One decent HE hit throws your aim off, so you're not firing back, and the next round is already sailing in. (I used to use the high ROF Sal-Kar howitzer, linked in three sets of two, could get perfect stability and by the time you've fired the third set, the first is loaded and ready again. Constant barrage!)

oh playing 6 on 6 in R6:Vegas and at around the same time my whole team ducks out...

Watching the nuke go off in COD4 was more of an "OH SNAP" than an "OH SHIT", but it's the same thing.
Also: watching Death AND Dracula come at me in Castlevania: Portrait of Ruin, realizing just how much HP Galamoth had in CV: Symphony of the Night (yes I'm a Castlevania nerd), realizing I had to fight Mass Effect's final boss with only one squad member (stupid glitches), any given moment from any given Silent Hill game, and the classic...

"Nuclear Launch Detected".

Haliniwali, that ZP:S mod rocks and my "oh, shit" moment is of course, being cornered (in the freezer slaughter room.)
Duck Sandwich - RE4 "Chainsaw? Oh shit! "quote" is still my favorite part of that game.

Iceman23:
Seeing/fighting my first Bloodsucker in STALKER Shadow of Chernobyl; I think anyone who has played the game can relate to me on that one.

Yeah that was some freakish shit. Also in the arena where you end up in the ring, fighting 2 guys with shotguns (or something)and all you get is a knife! somehow, meraculously i got 'em both my first try.

Alright, now for some of my own:
MOHAA Multiplayer - in a small room with one exit where some bastard American drops a grenade in my path.
Condemned - walking down into a subway, where you turn around real quick to a guy aggressively throwing you down escalator stairs! what the HELL was that for, man!?
Runescape - in the wildy: you think one guy's chasing you, but really 5 pure everythings light your ass up. "Noo!! my Steel armor and tuna!!"

When i was playing EVE-online (fairly long time ago when i was a noob). I was flying a shuttle instead of my Rax through 0.0 when i jump through a gate and the first thing i notice is the lock on sound, i try to jump away but they all have me webbed. So i sit back in my comfy chair, thinking " oh shit".

i've had loads others, but the only thing that actually get me to say it out loud.

CoD4 when you are playing free-for-all, you already have paranoia. You're crouching around looking over you shoulder and when you look back.... Click!...... BOOOM makes me go Oh SHIT everytime before the boom ^^

- Aeris. (FF7)
- The Big Revelation. (Bioshock)
- Seeing Boss I-No's boss-specific attack for the first time. Oh shit, balls! (Guilty Gear X2)
- When you first pick up the radio. (Silent Hill)
- The first bit after the training in Twilight Princess. What an underrated moment. Kind of freaked me out.

And the biggest one..

- Saying "no" to G-Man. (Half-Life)

Vampire the Masquerade: when "fighting" the werewolf...

Guilty Gear X: Final boss. You see a cute and kind girl playing with a squirrel, and you just know she's gonna kick your a** in less than 10 seconds (which she did).

Every. Single. Moment. of Shalebridge Cradle in Thief: Deadly Shadows. The knocking in the attic, the going down to the storm cellar to turn on the lights, the first time you see one of the "patients..." that place is a one-way trip down Heart Attack Lane.

-Every time the map expands in Supreme Commander and you find out that there's a massive army on the undefended rear of your base.
-First time seeing the Avatar or the Bloodthirster in action in the original DoW campaign. I actualy wasn't so intimidated by the squiggoth because it appeared at a point where you had a huge army and Gabriel just commented about rocket launchers being good against it in a realy calm voice. Both the other ultimate units show up at the 30 second mark and start rampaging through my scouts.
-End of Crysis mission 7. Floating through a hallway similar through the entrace, lighting flickers off and machinery powers down, i think "w00t, i'm gonna get my gravity back". Lights go on again and this time there's 3 aliens in my face.
-Dwarf Fortress. Post 3d version kobold ambush. Those who play it know what i mean.

I have another that just happened.

I was playing an old favorite of mine, Day of Defeat: Source. I noticed that the Axis were being quiet. I tiptoed over to their spawn to see noone, I searched and searched before I turned around quickly to see about twenty people crouch behind a wall. I thought "You sneaky little..." I looked, nothing. I heard a wooshing sound to my left and looked headlong into a rocket launched from the roof of a church. I sat for a second blinking before my head was blown to bits. I have never lived that down. I was able to pull that trick in reverse once, but it was darn difficult to get everyone to hide.