Happened to me just the once, with a girl whom, up 'til then I had been seeing regularly over 10 years ago. She was, I thought, about to slip the condom on but suddenly jumped straight onto me. I was soooo surprised, she had completed a couple of bounces before I extractaed myself and left. There had been no advance warning or previous suggestion whatsoever that BB was on the cards. I went to the GUM clinic, got the all clear, suffered abstinence for a period and put it down to experience. I did, however, notice just afterwards she started advertisng BB enthusiastically.