Mine happened yesterday on GW Crash Domination.
I started at spawn C as Marine Ops, moved towards the 3-story building, and randomly threw a frag over the wall towards the downed helicopter, this is usually just my little decoy that never really does anything. When I did it though, it must have exploded near a car (don't even know where there is one near the chopper) and it actually killed a guy.
I think it was "kewl" because I had no idea where it went and had no real intention of killing anyone.
That was also one of my most successful games, I downed 18 kills before getting killed myself by a 203, and also got the car bomb (possibly wrong name) challenge complete.