In order of "Holy %#$#@" magnitude
3) The game was Team Fortress Classic. I was a demoman playing defense on an Attack/Defend map. I had laid a pipebomb trap but then was chased away before I had a chance to detonate it. About 60 seconds later, I was defending the next point, and I wanted to make a new trap. I remembered I still had the previous trap, and I detonated it from across the map just for kicks. Quad kill.
2) Guitar Hero 3. Through the Fire and Flames, expert. I somehow managed to combo the entire intro of the song. 42 seconds in, I had a 500 note streak. That's the longest streak I've ever had on that song, and have not come close to replicating that. There are three parts in the intro that I usually mess up, and only have a ~2% chance of hitting, but this run I happened to hit all three. It was a random fluke, by all definitions.
1) Team Fortress Classic. I was a soldier defending on Dustbowl (DB on TFC required you to take a flag to the point, not just stand on it). My shotgun and rocket launcher were both drained, and I saw two pipebombs sitting on the ground next to me. I knew what that meant - the opposing demoman was going to use them to jump the flag to the final capture point. Then, I did something that, in retrospect, was probably not the recommended thing to do. I turned toward the capture point, and fired the one rocket I had managed to reload. Then I looked back and saw the demoman jump from his pipebombs, and by the most random stroke of luck, his jump put him in the path of my rocket and I killed him in the air. Now, hitting an aerial target with a rocket launcher is not unheard of, but if you read closely, there's one little detail about this story that makes it extraordinary: I fired the rocket before the victim went airborne.