It wasn't a scripted moment, but i was playing S.T.A.L.K.E.R: Shadow of Chernobyl, and i was hunting around this old abandoned army base for cool stuff. I had the Oblivion Lost mod, one of it's features is that there are random blow-outs (huge releases of radiation) every so often. So i'm skulking around and i hear this huge blast.
I run outside, and the sky is blood-red with swirling clouds and lightning all over the place. I look in my inventory and i have no radiation medecine. I start to panic and sprint to a rusted out bus in a field. I dive inside and crawl my way right to the back of the bus. You can be protected from the radiation of a blow-out if you get under solid cover, but a rusty bus with no windows or doors was hardly secure.
So i'm cowering at the back of the bus as thunder shakes my headphones and radiation starts to drain my health bar, i'm staring at the rusty floor because i'm literally in a blind panic, and i hear this faint rustling over the sound of the blow-out.
I look out of the window of the wrecked bus and i see a figure staring at me through the blurred atmospheric effects. It has hugely long arms, and it scoots around using them as extra legs. (Oblivion Lost also adds new mutants).
So now i'm trapped in the bus with only one exit, with the blow-out draining my health and defeaning me as a long-arm scouts around trying to sniff me out.
Now normally anything that is outside while there is a blow-out is killed, but apparantly this mutant had bugged or some shit.
So after a few minutes the blow-out ends, and i am miraculously still alive, although my radiation bar is high and my health bar is low, so unless i can find medecine i'm dead.
I step out of the bus, and the mutant is RIGHT IN MY FACE. I julmped out of my seat as he smashed my head in.
I have a few stories like that from various games, non-scripted, random events are always cooler.