Like, do you trip and just fall into someone's hole?
Not quite. I have to admit it has happened to me, but I was intending to fuck
someone, just not
that one. So the insertion was hardly accidental, but it was a case of a mistaken hole. Which, I believe, was the original poster's point.
You see, there are certain parties where a whole row of bottoms will line up on all fours, ass up, and face down. Given the generally poor mood lighting, and the fact that asses do not have nearly as many distinguishing features as faces, mistakes have been made. Sometimes the outcome has been quite pleasant, other times not so much.