Left 4 Dead 2 - Scavenge Bug
Leave bug strikes in the most epic moment ever, dooming 2 instead of just 1. Zoey gets deathcharged as the guy playing Louis gets votekicked off, so due to a glitch that Valve has neglected to fix for at least a year Louis gets teleported into the charger's clutch and Zoey's locked in place in her falling animation as the charger pummels Louis in midair.
Dear Valve, fix this already. I mean it's cute when stuff like this happens but I'd rather just play scavenge without game-breaking glitches.