Film Dope

A great episode starts with two great comedians barging into Central Perk and conducting a loud conversation on the couch, drowning out the chat between Monica and the friends. Pete insists on his quest to become the Ultimate Fighting Champion, despite Monica's reservations and also the fact he continually gets beaten up quite badly. Rachel cannot restrain her jealousy when Phoebe sets up Ross with a beautiful woman, and Chandler gets a few unwanted backside pats from his over-zealous boss.

It's my guess that Williams and Crystal were on the Warner Bros. set (who made Friends) whilst recording their movie Father's Day (also made by Warner Bros. and also at around the same time, in 1997). It's been probably the best cameo appearances on the show so far. Not only that, but the duo remain uncredited as well - brilliant.