Lucas, after wrapping up Star Wars, visits Spielberg on the set of Close Encounters. Impressed with the giant set and production and convinced Star Wars would be a flop, Lucas offers to trade 2.5 percent of each directors movies.
[youtube=https://www.youtube.com/watch?v=54i9-BYVvQA&rel=0]
Spielberg on Spielberg documentary, TCM
or just google “spielberg lucas bet”