Call of Duty 4, on the Russian village map. I'm in one of the two ruined houses, heading down the stairs when I get ambushed by some one turning around the corner. I do what comes instinctively, I knife the poor guy. However, he had martyr and I had no place to run back to in time, so I did the only thing I could think to do. I picked up the grenade and chunked it out the room and into his friend's face.
Another CoD4 moment, though more observed embarrassment, was on the warehouse/building thing map during a Search and Destroy game. I was defending, and killed all but one guy on the attacking team without moving. I wasn't in a ghillie suit, I was a Russian with an assault rifle as my primary, just laying prone in the storehouse and looking over the objective. The attacking team would walk right over me, look back at me, everything, but still never shoot at me or seem to notice me. They then got very angry with me over the whole thing.
And last little epic moment, TF2. I can't exactly remember the map, but it had two approaches. One a long winding path through a quasi-city, another through a back alley/sewer type deal. I was a pyro and decided to check the sewers. I saw some one coming, pulled back, and waited. I then jumped out and turned my flamethrower, causing the incoming heavy, demoman, and scout to run. I'm really not sure why, but hey. They were apparently terrified of me.
That particular moment was balanced out in the next map, the train depot one. The game had just started, we were all charging out of the gates, and I decided to wait near one of the entrances on a track while we captured the depot part. My decidedly 'bad ass' attempts at guarding were completely negated when the train ran me over.
In Halo 3, and I'm sure I share this moment with many, is mistiming throwing a grenade through the little plasma shields in Snowbound. I loved abusing the little shields when I was outnumbered, and once decided to try my luck with throwing a plasma grenade out at the two guys ganging up on me. It took me a second after I had stepped back inside the building to realize that the plasma grenade had never left the premises.
EDIT: An observed case of epic as well. A friend of mine had to show me a video in Halo 3, sand trap, one flag CTF I believe. He was driving in the Chopper and the enemy had the flag in a Warthog. He's coming down a sand dune straight for them, guns blazing when I imagine he realized he was a bit too close to the enemy's elephant. He does this slide thing, slipping underneath the warthog. He launches it into the air. Just in time too, the warthog catches a rocket that was intended for his chopper.