You know, one of those races where you go out too fast and end up practically crawling to the finish line. Or, your pace is fine but for some reason you just bonk and the race becomes a death march.
My worst was probably my first 5k race a few years ago; 2002 I think. Long story short, I hit the first mile in 5:40 and finished the race in 21:40. Ouch. The worst part was when I was eeking my way up a hill around the 2 mile mark when a chick passed me and said, "Good job!". That was the first time I was beaten by a girl in an athletic event. It completely demoralized me.