I'm a fan of Friends and I thought I'd seen all the episodes over the years but I was in the gym the other day and it was playing on one of the TV's, and it looked like an episode I hadn't seen. The only info I have is that it involved Monica on stage doing what I originally thought was karaoke but I think it might have been a stand-up routine or something. Chandler and I think Phoebe were at a table watching her.
Thanks to the quick shot I caught of a scene w/ Jennifer Aniston, I can say the ep is definitely not from the earlier seasons. But that's all I got.