I don't think I ever had that happen to me but there was a single time where I was getting off using a playboy my brother hid while laying down on my bed, under my covers. My older sister suddenly barged in my room so the only thing I could do was prop up my legs to hide my boner, my hand movements and cover the magazine at the same time. I could never tell if she knew what I was doing, but if I could do it again I might of let her see just to see her reaction.