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.
One thing we can all learn from AirBnB: if any group of people you hire for any reason, even on a contract basis, do one bad thing on the Internet one time, haters on the Internet will never let you live it down. Hire carefully.
You don't do handshake deals with people whose fundamental business model is "cutting corners wherever you can sneak away with it"