This one's another lame one. It comes to us from the absolutely brilliant game Tetris (specifically this version). It's more a personal bragging right, and it's not really all that impressive, but I was pretty damn impressed at the time, and this list is a lot more nostalgic than the other one.
For the longest time, level ten on the Microsoft Entertainment Pack version of Tetris was essentially a death zone. I could get there with little effort, but once I got there, it was just a matter of time (usually short) before I'd make a tiny error, and the pieces would stack wrong, and I'd be buried and lose. Everything just happened so fast, and the margin for error was just so small.
Then one day, I got in the zone. Everything felt zen - I was adjusting to every piece with ease, leaving plenty of room for whatever the game would throw my way. I wasn't just playing a good game of Tetris; I was playing, by far, the best game of Tetris I'd ever played.
Eventually, as it always does, the moment passed, and I lost. In the heat of the moment, I had obviously been unable to keep tabs on my score, but I knew it was going to be good. I glanced up to see how I fared, and to my amazement, saw a negative number. I had gotten such a high score that it overflowed the counter and spilled negative.
Much like the last moment, I haven't tracked down the version of Tetris in question, and none of the few screenshots to show a negative score are mine, so they don't really mean anything, but the shock of breaking the score counter, combined with the absolute zen-like feeling I had while playing that game is an excitement I've never really surpassed (with a couple of exceptions, but they're higher up on this list, obviously).