My whole time in FRC has been a parade of "worst experiences during competition." Poorly timed mechanical failures, strategic mistakes, bad scouting, untested code, communications errors, hard to diagnose problems, etc. It's hard to pick just one.
This one probably is the easiest to translate into this thread. To provide a little setting, this was the very last qualification match of the 2008 Philadelphia regional. We were currently on the bubble to be an alliance captain, so we were getting prepared for the eliminations. Our software lead had loaded some new autonomous code onto the robot to correct one issue, but apparently decided to make another change as well. You can see the results for yourself.
http://www.youtube.com/watch?v=EESb452FMHw
To add insult to injury, in a penalty plagued 2008 game, we had yet to take a single penalty. This failed automode was our first.
It turns out we ended up being the #8 alliance captain, which not only meant we had to face the vaunted combination of 103 and 272 in the quarter-finals, but we'd be playing in the very first elimination match of the afternoon. One very very hectic lunch break later (in which Chuck ate all my pizza), our "spare" claw was mounted and we were essentially ready to compete. We just ran laps and played defense in the first match, since we weren't confident in our claw repairs yet. In the second, we managed to actually hurdle once again. Not that it really mattered, as we were plastered by the top seeded alliance. Who knows if we would have been on a higher alliance (and avoided the powerhouse #1 alliance) if our claw wasn't split in half.