Starring: Jennifer Aniston, Courteney Cox, Lisa Kudrow, Matthew Perry, David Schwimmer, Matt LeBlanc
The One With Rachel's Big Kiss - S7-E20
Other mistake: Matthew Perry supposedly really likes wearing a tux that turns out to have been made for Diane Keaton. Except that Diane Keaton is 5'7" and Matthew Perry is 6 feet tall, and also considerably larger than the very slim Diane. He wouldn't even be able to put it on.
The One Where Phoebe Runs - S6-E7
Visible crew/equipment: When Phoebe bends over to tie her shoelace before the first run, and later when she surprises Rachel at the park while running, the mic pack bulge under Phoebe's top can be seen on her back, and when Rachel bends over there's a bulge at her back as well. It's also a bit noticeable the third time they run.
The One With Ross's Teeth - S6-E8
Monica: What happened to your teeth?
Ross: I whitened them.
Chandler: Really?
Ross: Yeah, what... What do you think?
Monica: Well, uh, I think I shouldn't look directly at them.
Ross: Come on, seriously.
Monica: Ross, they're really, really, really white!
Chandler: Yeah, what was wrong with your old... Human teeth?
Ross: Well, I did leave the gel on a little longer than it said to.
Monica: How much longer?
Ross: Uh, uh... A day.
Monica: Ross, you know that tonight is your date with Hillary?
Ross: I know! That's why I did it! Come on, are they really that bad?
Chandler: No. No, no, no, you'll be fine. Hilary's blind, right?
Monica: She will be after tonight.
Trivia: Matthew Perry's wit is so legendary that the scriptwriters have often incorporated his gags into the show.
The One With The Jellyfish - S4-E1
Question: When Ross and Rachel are fighting, Chandler hides behind the door and bursts out saying, "I knew it!" When Rachel says, "It's not that common, it doesn't happen to every guy, and it is a big deal!" is this just a Chandler moment or is there another joke I have missed?
Answer: The statements that she's contradicting (that it is common, it happens to every guy, and it's not a big deal), are the things that a woman commonly says to a man who is suffering from erectile difficulty, typically to assuage his bruised ego. However, most men do not believe that these statements are true, as evidenced by Chandler's outburst. He's so caught up in the proof that women are lying about it that he gives himself away.
Rooster of Doom