Two nice glitches I came across by accident:
The first, in L4D, me and my buddy were playing versus or whatever and we were the infected, on the farm level if it matters. It was the final chapter thingy, and anyway, I spawned as a tank close to when the rescue tank thing comes for the survivors. I had some fun, and then a few seconds later I heard my friend yelling, so I turned around and - oh dear, another tank. We absolutely destroyed the other team, and they would have completed the mission-dealie if it wasn't for the double tank thing.
Anyway, second glitch was in COD4 online. I've heard it happens from time to time, but not that often (I guess). I was with like 5 friends or something and were looking for a lobby to join, and I didn't get into the one my friends did. I waited for them to finish their game, but they said everything was going in slow motion. I got into the game, and it was indeed in slow motion. Everything, the timer, movement, shooting, were all in slowmo. It was awesome. We cranked up the sensitivity settings on the options, so we could aim at top speed and still be going slowmo. We destroyed the other team; I doubt they all thought to do that. The best part was it wasn't the GAME that was slowmo-glitched, it was the lobby. We had about 6 good matches like this