Your Worst, Most Unexpected, Startle Scare in a Non-Horror Game?

So I'm talking a non-horror game here. Everyone who's playing the Resident Evils, Dead Spaces, and Fatal Frames knows that something is out to get them. You are constantly on edge and the games listed previously do their best to build that tension constantly. The real art, here, is crafting a moment that scares you in a game that is otherwise not even trying to. It's almost even more effective to try and execute in a game that isn't scary if only because it is so unexpected. The more pleasant and mellow the game, the more embarrassing this is. So, without further adieu, here's mine:
 

It probably involved a cougar and/or bear in Red Dead Redemption. 

Don't know if it qualifies as a scare but i was pretty freaked out when you shove that fang-shaped knife in that dude's brain in Baikonur in Black Ops.

"Check Cable" screen from Dr. Robotnik's Mean Bean Machine from Game Gear. 
 
Thinking back on it, I'm not sure if it was even real or just a nightmare, but that thing used to scare the hell out of me. So much that I stopped playing it.

All the time in multiplayer games. I always get startled when someone comes around a corner, and I'm not expecting it. 

" "Check Cable" screen from Dr. Robotnik's Mean Bean Machine from Game Gear.  Thinking back on it, I'm not sure if it was even real or just a nightmare, but that thing used to scare the hell out of me. So much that I stopped playing it. "

Playing the hardest difficulty in MGS3 where getting caught resulted in a game over. I was crawling through the grass when a soldier was behind me and spotted me. The exclamation point sound scared the shit out of me because I was too focused on being sneaky and quiet.

Knife ending in 999 (I even made a thread about it).
 
Also, playing as naked Raiden. For several reasons.

The music that plays when you first enter the castle in Mario 64.

" It probably involved a cougar and/or bear in Red Dead Redemption.  "

" The music that plays when you first enter the castle in Mario 64. "

The first time I accidentally got caught and heard the exclamation noise in MGS1, MGS2, MGS3, MGS:PO, MGS:PW, MGA, MGA2 and MGS4
 
It's something of a tradition.

" It probably involved a cougar and/or bear in Red Dead Redemption.  "

This scared the shit out of me when I was a kid:
 
  

Various headcrab incidents in Half-Life 2. 

In metroid fusion the thing that took your suit would always appear out of nowhere and you'd have to hide, and if you didn't (or you hid bad) it would kill you. always scared me really bad.

Id say honestly just being on the beaches for the first time in Medal Of Honor Frontline was more rattleing than almost any other experience in gaming at that time 
and idk if it counts but Bioshock scared the fuck out of me sometimes  
@EvilTwin said:

" Various headcrab incidents in Half-Life 2.  "

Far cry 2 jeeps

" @TheSilentTruth said:

" The music that plays when you first enter the castle in Mario 64. "

The eels and pretty much any underwater level in Mario 64. Having the calm, tranquil water music playing when an eel jumps out at you is unsettling. "

In the past two or three years - Fallout New Vegas, when near the end of the "they-went-that-a-way" quest, Victor will suddenly just appear behind you and initiate conversation. That made me jump - you're just walking around the mojave wasteland when suddenly "HOWDY FRIEND!". 
 
Apart from that - COD4 - the dogs - especially when you're too concentrated on something else and you don't really hear them coming and suddenly "BAM" you're down. I hate those dogs so much.

The void, I wouldn't call it horror but literally half of the characters are fucking terrifying. FOR EXAMPLE:
 

Whenever a Metroid appears for the first time in a Metroid game. Those things creep the fuck out of me. 

That damn AI in Mass Effect 2 Overlord DLC screaming. Goddamn I had my headphones on and had the sound up pretty high. I almost jumped out of my chair.

I was playing Oblivion, sneaking in the wilds of Cyrodiil towards a deer, approaching for the kill, when I suddenly, out of the blue, was spoken to by an NPC that was delivering a letter to me. 
Now I'd been sneaking for some time as I just wanted to try for authenticity just this once, and this very one time that I am going for authenticity this NPC pops out of nowhere and says: "You there! I have a letter for you!" (Or something along that line.)
Frightened the burjeezers out of me so much. And needless to say that the deer noticed us and ran off.

" It probably involved a cougar and/or bear in Red Dead Redemption.  "

The poison-headcrabs from Half Life 2...

" That damn AI in Mass Effect 2 Overlord DLC screaming. Goddamn I had my headphones on and had the sound up pretty high. I almost jumped out of my chair. "

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SPOILER WARNING: Click here to reveal hidden content.

Anytime those gnomes took a puzzle piece in Puzzle Agent.

"

Getting randomly shot by raiders/super mutants in Fallout 3, especially if you have headphones on. 
 
Be walking along, listen to the calm music, no red triangles on mini-map then BLAM

If it counts as a non-horror game, the dentist/some kinda doctor in Bioshock. I jumped in my chair the first time. Too bad it was scripted. 

" It probably involved a cougar and/or bear in Red Dead Redemption.  "

The first time you run into a werewolf in OoT I quit playing for the rest of day. Those things are terrifying.

The husks in ME2. When you get hit by one, your feet are glued to the ground. 

During Hitman: Blood Money, when that crazy bitch dressed as an angle brutality stabs you like 40 times. That crazy cut sense comes out of no where

Ninja Gaiden on NES...  I was looking for something and when I turned my eyes back to the tv, the face of Jaquio burned into my memory forever... I ran like hell to outside.  That picture got me pretty scared when I was a kid. @ 6:30.  
Still makes my heart pound faster than the heart of a hummingbird 
 
  

I distinctly remember a moment in Half-Life 2 where I was walking down a dark tunnel and a fast zombie jumped up into my face, screeching as it went. I must say, it was a little unexpected.

Creeping around in Fallout New Vegas and then standing on a bear trap/ trip-wire shotgun trap *HEART POUNDS*

Almost every enemy in minecraft has startled me at some point. I will be exploring a cave very carefully and SWISH PLUNK an arrow comes out of the dark and nearly takes me out. Or I am walking underneath a hole from the surface and all of a sudden, SHHHHHHHHHHHHHHH BOOM a creeper comes out of no where. It is a terrible brutal world in minecraft. 

Back when Morrowind came out, I knew nothing about the game or it's predecessors. I went into it vanilla and got a fresh experience. The combat was easy and I killed everything that moved. Then, one day, I was running around a temple. Everything was musky, dark and gloomy. I come to this corridor, and look down the hallway. In the end, is a dark, wide shape. It turns to me and it starts moving. It's coming closer, and as I pee my pants I can see it's horrible features unfold before my very eyes.

Hell yes I ran. And from there on I've just been scared shitless over every Ascended Sleeper I've seen in the game. They just creep me the hell out.

The intro to Strider actually scared me witless as a child. As soon as I pressed start, I didn't realise I would hear that high pitched, 16-bit laugh through my speakers :-s

Batman Arkham Asylum's Scarecrow parts. The first one, where you're in the morgue, confused the shit out of me because I didn't know I had to investigate the corpses. So first I walked around in there with that pressuring windy sound pushing my ears, and then I investigate the first corpses (moma and pops) and then I decided that what the hell, this is probably just for atmosphere. So I walk out the door and BOOM another fucking morgue. I just do the same thing again with the body bag and WHAT THE SHIT FUCKING SCARECROW OUT OF FUCKING NOWHERE. My heartrate had already built from walking around with that sound on loud and when this shit happened I jumped so much I had to pause and get something to drink. 
 
I mean, I knew Scarecrow was gonna be somewhere around there and I was pretty confident Jim wasn't dead, but I was so dulled out after having walked around for so long that him popping out like that really hit home. I think the sound really played a major part in building it up though. 
 
Also, dem zombies in Mass Effect. DAT FUCKING SOUND.  
 
I really shouldn't play video games with headphones on. I'm quite confident I could piss myself playing pokemon or some shit. Playing Metro 2033 is a fucking pain because it also has dat sound which makes me feel uncomfortable because I know some shit will happen but I have no idea WHAT it will be.

mine craft.... fuck you spiders

The elevator scene in Half-Life 2: Episode 1. Still one of the best encounters in FPS history.