Well, the younger sister of one of my girlfriends walked in on us, while we were having sex. Or more precisely, she seems to have heard us, got up at night and checked, what the noise was. Then she just stood in the doorway, rubbing herself between her legs. But when I made eye-contact, she ran away. She never wanted to talk about it, so we just pretended it never happened. And my girlfriend probably never even knew.