Heh..I've got quite a few, all with my best friend.
First in Diablo II: We were in Nightmare difficulty, Act 2. While exploring a tomb, I ran ahead of him into a room and I heard a distant "BSHFT". Next thing I know, I see
Xadow has been slain by vase. He got owned by a Nova trap in a vase at level 45.
In Gears of War: We were at the part where it's storming and got to the cart ride. Every single time, one of us ended up dying somehow, due to a missed shot or getting surprised. It took us almost an hour, nearly making us give up Insane difficulty.
Also in Gears of War: While fighting General Raam, we must have died about a million times (Again, on our Insane run). Eventually, we both made it to the other side of the train car, as we thought we saw a mountable machine gun turret on a ledge. As we made our way around the back, he fired his shotgun in Raam's direction. It killed him.
Ghost Recon: Advanced Warfighters: For some reason, bullets seemed to home in on his head, because every time he'd peak out around a corner, he would drop dead and promptly spam the chat window with swearing. At one point, we were on the second mission and at a corner leading to a plaza. He peaked around and got shot in the head. He came back and got shot in the head at the exact same place. We finally managed to kill the machine gunner doing it, and he got behind a car. Bullet through the car window, into his head. I was howling in laughter at that point.
Splinter Cell: Chaos Theory: Probably the funniest thing that's ever happened to us. Well,
me. In one of the coop missions, we thought we had cleared the entire level, so I ran down a hall to meet him and get to our objective. Suddenly, I heard a loud bang, the screen freaked out, and I died. A guard had slammed open a door just as I ran by, sending me flying into the opposite wall. He laughed so hard, he couldn't properly deal with the threat and got shot down.