Playing the original Halo on co-op. We were playing through the level where you are in the snowy part of the Ring. You exit out of a tunnel and climb up some rocks where a bunch of Elites, Grunts and two Ghosts are waiting for you.
My pal ran off to the left, blasting with his assault rifle at one of the Ghosts, while I ran to the right, blasting enemies with my shotty. I glanced over towards him and saw that he was getting out-flanked by the two Ghosts. In response, I pulled out my rocket launcher and fired a shot at the Ghost while on the run, smacking it center-mass and sending it flying.
The kill was on my buddy.
After I hit the Ghost, he gave me a cheer ("Awesome shot, dude!"), before turning from happy to sad very fast ("Oh, sh--") as the Ghost careened end-over-end toward him. He couldn't get out of the way fast enough and was quickly flattened by the large piece of scrap, amidst a string of curses at his bad luck.
I laughed so hard at the spectacle that I ended up dying too. So priceless.