When you think you are by yourself so you start doing Bruce Lee karate moves only for someone to be looking at you thinking "the fuck you doing??
Or my one, when I was at Tafe (college) I was showing my friends this humping technique I seen in a book on top of a table which was for some reason on top of another table. Anyways we were laughing cause it was funny as fuck and as fate would have it the top table slipped from the bottom one, fell and I fell with it JUST as a large group of girls were walking past...
I didn't get over that for about 2 mins.
peace.