Following on from Nina's Best Sex thread, it reminded me of the most surprising sex I've had.

Happened about 10 years ago, I was with a group of friends including one of my best friends and her younger sister. As the night went on everyone went their separate ways and I was left with the younger sister. I offered her a lift home, and as we were driving her hand reached over and started to tease and rub between my legs, before unzipping my fly and proceed to give me a BJ. It took all my concentration to focus on the road until I could find a quiet side street to pull into, where we continued the fun and games.

Not the best sex, but definitely the most surprising. There was no hint of this ever happening before or since. It is our little secret.

So what's you most unexpected or surprising experience