The One with Two Parts: Part 2 - S1-E17
Other mistake: Rachel is limping in the hospital for the first time. When she gets back home, she walks perfectly fine. Then in the hospital, she starts to limp again.

Starring: Jennifer Aniston, Courteney Cox, Lisa Kudrow, Matthew Perry, David Schwimmer, Matt LeBlanc
The One with Two Parts: Part 2 - S1-E17
Other mistake: Rachel is limping in the hospital for the first time. When she gets back home, she walks perfectly fine. Then in the hospital, she starts to limp again.
Visible crew/equipment: During the scene when Phoebe breaks into the other Phoebe's house and gets caught, the older Phoebe was telling the younger one that the man in the photo is not Frank Buffay, but is Chuck Mangioni. Right after that, as Phoebe 2 walks towards the camera, you can see that the door behind her moves open, and there is a visible shadow of someone who walks by that doorway. (00:19:00)
The One With Chandler In A Box - S4-E8
Monica: Fine, judge all you want to, but [to Ross] married a lesbian, [to Rachel] left a man at the altar, [to Phoebe] fell in love with a gay ice dancer, [to Joey] threw a girl's wooden leg in a fire, [to Chandler] LIVING IN A BOX!
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