Huh, strangely glad to know that we weren't the only one to be turned off by a frisbee. We had it happen one time when our human player was trying to load our robot.
Here are what I remember as the worst moments, for various reasons.
2012 Rebound Rumble - Trying to balance on the bridge, accelerate too fast, begin falling backwards, and bend a bunch of stuff. None was very critical, but a damaged robot is never fun. Video used to exist at TBA, but it's not there right now.
2011 Logomotion - At TC I think, we managed to get our lift stuck on the scoring rack, and that was quite distressing for how long it took to get it unstuck.
2010 Breakaway - The whole robot. It's like it was anit-designed.
2009 Luncay - We were in the quarterfinals, and we were up one game to zero at TC (
http://www.thebluealliance.com/team/857/2009). The second match, we had a bar protecting our intake roller that was ziptied on because our previous one had broken off. However, the zipties failed, we exceeded the allowable dimensions, incurred a 10 penalty, lost the match by 4 points, and went on to lose the third game by 3 points (sooo close to semis....).