This was back on Counter-Strike 1.5 (before Steam came out) and was at a local BYOC in Sydney.
My team was losing 12-5 (first team to 13 rounds wins) on de_inferno against one of the best teams in the state and we had no money whatsoever having lost the last round, so I used all my money to buy a desert eagle and a couple of flashbangs. In desperation, I rushed the small bombsite and threw my flashbangs down the banana, where I proceeded to run into the rest of the enemy team.
After I threw my flashbangs, I found I'd caught the whole team at the end of banana. I kept running and started firing, with my first three bullets getting three headshots. A fourth guy was stuck at the back and he was quickly dispatched with a fourth shot. Unfortunately I wasn't recording that day, but it's easily the biggest thing I've ever done and to do it in the heat of competition was phenomonal.