...when your loading into a BF2 map and the sever note stats base rape is ok!

... when the next room you enter is really big and round like an arena.

... when the boss has several weak spots that all have to be blasted ... or one minuscule weak spot hidden on his backside.

... when you're fighting up close in a small room and accidentally lob a grenade instead of switching to the shotgun.

... when you hear laughter of any kind (e.g. insane ally, some demon-monster, ghost of a little girl, evil incarnate etc.)

... when you see lots of fire.

... two words: dead scientist.

in HL2
...When you hear a familiar little shriek...headcrab!
...When you hear running and insane laughter behind you, coming closer at breakneck speed...fast zombies!

In general
...when your little cousin runs around the room and trips on the controller cord and you're about to fight a boss.

...when you're taking a command post and hear a dozen spawning *clicks* around you... (Starwars Battlefront)

...you see a Serious Backpack (Serious Sam)

i got one.
when u twitch on the sniper and miss the persons head(Halo2)

...when your mother calls you for dinner.

hahaha how bout

when your the only one left on your team agaisnt six other people and all you have is a sniper rifle and a handgun with no ammo or a different gun anywhere near you and if you move from your hiding spot your sure to die.....

moaning of any kind......

a blood trail.......

you hear a click.....

the door shuts behind you.....

and finally sometimes water dripping.

oooh and broken glass or glass breaking

OMG WTF FILE CABINETS! (Counter Strike Source)

In Enemy Territory, whenever someone fires a Panzer rocket, it makes a high pitched squeal right before it fires (it takes a few seconds to fire) that EVERYONE can hear.

Its great. Some of the people I play with are so paranoid about that sound, that if they hear someone similar in real life, they might jump