Worst I can remember was in my final year at high school, at an inter-school swimming carnival. We had to wear speedo-style swimming gear, and like the conceited 16-year-old that I was, I went to buy a drink at the food kiosk wearing just my swimmers - I had a good body and I didn't see why I shouldn't show it off.
Anyway, the two girls serving in the food kiosk were just to die for - both blonde, possibly sisters. As soon as I saw them I knew I was starting to rise to the occasion, but when one of them turned round and bent over to get something off a low shelf, that was it. There was a line behind me and a long walk down a slope back to anywhere where I could make myself inconspicuous, all with a boner that felt like a telegraph pole and me wearing virtually nothing.
No one said anything, except for one of my mates who saw me and thought it was hilarious - naturally - but I remember several teachers from some of the other schools giving me horrified looks. I guess I showed off more of my body that I had bargained for!