Lucas entered into a wager with long-time friend Steven Spielberg during the production of Close Encounters of the Third Kind. Lucas was sure Close Encounters would outperform the yet-to-be-released Star Wars at the box office and bet 2.5% of the proceeds of each film against each other. Lucas lost the bet and to this day Spielberg is still receiving proceeds from the first of the Star Wars movies.
— source: http://en.wikipedia.org/wiki/Star_Wars_Episode_IV:_A_New_Hope