Mine is when I was trying a dare to run nude over a railway footbridge.
I did it at night, checked lots of times to make sure nobody was around then stripped totally naked.
I ran across the bridge (4 tracks worth) to be met by a bloke at the top of the stairs. He was quite angry and made me rather worried. Suffice to say I ran for it.
Probably explains my worry about being caught naked and why I REALLY want to take part in one of @Ed s outdoors naked experiments.