Today was the Shikkerdovid’s wife favorite talmudic incident. Upon belief, this scenario is mentioned two times (maybe more, but these are the ones the SD remembers). A man is on the roof, he falls off the roof into (sexually speaking) a women. According to Rabbah he owes her damages, but does not complete Yebum. The SD fondly remembers when he learned this in Yevamos for the first time and came home to tell the wife. She freaked. The SD rembers the her words fondly, “this is what you do every morning rather than help me with the kids.”
The serious question is why is a man owe damages, but not complete his yebum requirement? I leave the floor open. One guy out there, “RH” has the answer. I leave it to him to post.