I've got two:
In high school I waited till the back stretch of the last lap of the mile to pass a rival runner in front of where my team was camped out. I pulled even with him, waved at him and loudly said bye bye. I was confident because he'd never beaten me...until that day. Doh!
I entered a sprint triathlon soon after finishing my college senior year track season. I was in the best shape of my life. I wasn't much of a swimmer, but could cycle well, and obviously ran well.
So I entered the elite division because surely as a decent college runner this triathlon thing would be simple and if all else failed I could catch them on the run
I realized my mistake in the transition area during the set up time...with my 30 year old bike with strap on pedals next to the all carbon, aero bars, and specialized helmets. The elite wave started the 800 meter swim 5 minutes before everyone else so people could watch the "elites" go at it. I was passed in the water by people that started in the next wave.
To make matters worse, it was part of a triathlon series. I had already registered for another race in the series. They required you to stay within your original classification (elite vs open) for points purposes. So I got to relive the misery a couple of weeks later.