Red Faction: Gurrilla.
i Headshotted an APC driver with the EDF rail sniper as he barreled towards the building and rebels i was defending. the APC, however, had enough momentum in it to hit the small wall i was using as cover, Misty Flip through the air, and broadside the very building i was defending before it exploded catastrophically. taking out all of the front side the building then collapsed forward onto not only myself and allies, but also the EDF attacking forces.
my computer lagged for a full 3 minutes as the 4 APCs, and 30 some odd people, myself included, where crushed this falling high rise as a cascade of explosions engulfed everyone.
i survived of course because i was Mason, Red Rock God of The People, but unfortunatly i failed the mission because all the rebels died.
all of that from just 1 sniper shot.
no other game i've seen has had an unscripted sequence like that save. and the one game that did have that (ArmA2 Multiplayer) was no where near as epic.