I probably have more proud moments than this one, but this is the one I remember the clearest.
It was a slow day over the summer and I got a steam message from a friend that they needed a 4th for their L4D2 round, and they were already halfway through the map (Hard Rain 2). When I joined the game, they were getting curb stomped. The score was something like 1200 to 100. I still don't know exactly how I did it, but the craziest thing started happening. We didn't just start fighting back, we started winning. I somehow managed to bring us back.
By the end of the match, it came down to the finale. We were slightly ahead, so we went first, with only one of us making it to the rescue boat. Then came their turn. They had one guy that, no matter what we did, we just couldn't kill. Eventually, I got the message that I was about to become the final tank of the round. The only problem was, I spawned all the way across the map, and he was standing in the boat area. If he and the 2 guys he was with made it on, we were done for. So I took the one dumpster in the entire level and whacked it randomly at them. Over the building, it hit all 3 of them, game over. We won by 1 point. I think I still have that screenshot somewhere...