This happened to me once!

I was on top, sorta missionary but with her legs folded up to her chest. I was doing what wet called the "Rabbit"... self explanatory.

I was going as fast as possible with maximum thrusting, it was going great, she finished twice. Apparently I over stroked and I went balls deep in her ass, came out and went right back into her vagina, in the same stroke.. She screamed bloody murder and smacked me in the mouth. We weren't strangers to anal, she loved it actually! But that time, no preparation, no warning. We never did that position again. She was crying for a while after too.