i learned Bruce Willis appeared on Friends for free after losing a bet to Matthew Perry
Apparently Perry was quite the gambler. He got into a debate with Willis while the two were making The Whole Nine Yards. Perry believed that the film would be number one in the box office on its opening weekend, but Willis disagreed. In February 2000, the film was number one. Willis was set to appear on Friends as the dad of Ross’s girlfriend and Rachel’s love interest. As a result of the bet, he had to donate his earnings for the guest stint to charity. (x)