Docs were finished, a "closing dinner" was had, a handshake was made on the deal, and then AirBnb backed out. If an investor pulled that on a YC company I'm sure they would be blackballed rather quickly.
You don't do handshake deals with people whose fundamental business model is "cutting corners wherever you can sneak away with it"
This is how Visa works https://news.ycombinator.com/item?id=4396414 (900+ points 3 years ago)
"Docs were finished, a "closing dinner" was had, a handshake was made on the deal, and then the investor didn't sign"