YMMV / Friends S 3 E 24 The One With The Ultimate Fighting Champion

• Big-Lipped Alligator Moment: Robin Williams and Billy Crystal's guest appearance at the beginning of the episode has no relevance to the plot and is never mentioned again. It wasn't even in the original script, they coincidentally just happened to be in the same building where the show was filmed and the writers asked if they wanted to make a guest appearance. Lampshaded by the gang when it's over.
  [cut to the gang, they’re all stunned]
  Phoebe: So, Monica, what were you gonna tell us?
  Monica: (pause) I have no idea.