When going at it like dogs, the penis, moving quickly, then ejects from the vagina and enters the anus, often resulting in pain and extreme terror/pleasure.
Often to calm the victim of this "surprise", the penetrator yells
"SURPRISE BUTTSECKS!!" in a vain attempt to make light of this magical , yet, often horrible experience.
Man: Oh yeah baby
Man: opps i slipp.....
Woman: Ow. OWWWWW!!!!!!!
Man: Surprise Buttsecks!!!!!!
Woman: this date is over! Bus driver let me off here!!!!!!