My brother and I were at a nudist jetty in Vienna (long story....) and I decided to jump in the lake for a swim and was trying to get my brother to come with me, but he was being really reluctant, which surprised me since usually he's even more keen on swimming than I am.
Anyway, finally he got up and I saw that the reason he hadn't wanted to was that whilst he was lying on his front, he had been able to hide is semi, but when he got up it was plainly visible.
I don't think anyone saw it, but he was quite embarrassed about it..