When two or more men are engaged in some sort of testosterone-based activity (I.E. wrestling, football, rugby, etc...) and there is incidental penis to anus penetration. The term "accidental anal" refers to that occurrence.
"Matt and Luke were playing a heated, but quality game of chess that ended in accidental anal."
"I just wanted to put a dollar in the homeless man's tin can, but accidental anal occurred, too. Now I need special medication from the clinic."
"The poker game ended much as it started, with accidental anal. I think we're all walking a little slower this morning."