GTA IV: A friend and myself decided that we wanted to try online freeplay. The idea was to see how long we can survive a police chase with one of use driving and the other firing at the cops. I the first to drive with my friend in the shotgun position, literally, in my choice of vehicle for this type of gameplay. A nice, heavy truck.
After about five minutes of the cops chasing us, I noticed another player had joined the game at some point. We had forgotten to password the game, so anyone could join. Granted, this was the PC version so there weren't many players looking to play online. Thinking nothing of it, we continued on.
Sometime later, we had collected a good number of cops and my friend -- who had unbeknownst to me, decided to take a few shots without her gun-- was firing drunkenly at the police. I was focusing on the road ahead when I spotted a motorcycle in the oncoming lane jump the median and head right for me. I swerved, taking a glancing blow and sending the rider flying. I look behind me, only to see this flying person smack right into the oncoming police and cartwheel over my pursuers. Then, his skidding bikes hits the lead cars, sending them flying. I slow down so I can continue to watch as a few of the police cars flip and the others pile up. I assume what happened next was my drunken friend got a good shot off. A cop car exploded, followed by another and another until the ten or so that had been chasing us were nothing more than several charred wrecks in the middle of the street.
The other player, after seeing what we were doing, decided that he wanted to ruin our fun by wrecking us. In a fit, he leaves the game. Just to top off the situation, my friend pipes up with "What the bloody hell was that?!?" over voice chat. Unable to answer, I sped away laughing so hard there were tears in my eyes. The thing is, I couldn't see where I was going now and after a few seconds I hear my friend yell "look out!" just as I plow into a lamppost, killing us both.
No other unscripted moment has ever come close to that.