I have several.
First, that moment when fighting a particularly hard boss, who is about to kill me, as I realise that my last save was 3 hours ago. Fuck you, Dead Rising 2 and your psychopaths.
Having a spider in Minecraft jump on my head from a tree, while I'm completely unawares that it's there. That shit FREAKS ME out... -.- AND their eyes glow...
Playing a fighting game online, or with a friend. I'm trouncing the guy, and they begin to make what seems to be an unavoidable comeback
Serious Sam. Whoever thought up those skeleton, horned thing is an arsehole...
Killing Floor. Last zombie of the wave, we all make our way to the trader, and it turns out that last zombie is a FleshPound...