Two feats that come to mind:
1. Need for Speed Underground released and my older brother had been playing it non-stop. He wanted to race me but I wasn't as far in the game as he was so he let me use one of his cars. A car that was slower than his nissan skyline (his prized possession). Somehow I wonder the race without any dirty tricks. After my celebration my brother silently left the room and said "nothing big it's just a game." No sir.............. That loss hurt your soul........ And I will bask in the glory
2. Beating Crash Bandicoot one, with the terrible save system and frustrating levels the fact that child me beat that game still surprises me. Last year I replayed CB and I wondered how I had the patience to finish the game (and back then I used to get very angry and yell at my TV like I was going to murder it). Maybe because back then every game was a platformer and I've now gotten rusty but I personally think that at least the original CB is pretty tough.
Bonus one: beating all my friends in a round of Super Smash Bros on N64 with the broken N64 controller. They were much better than me and played with top characters, I had a controller that was a poor excuse of old nintendo technology, I was pikachu playing Nes, Samus and Kirby