The One Where Monica Gets a Roommate - S1-E1
Revealing mistake: As Monica is walking to her room and steps on Paul's watch, if you look carefully, you can see that she just steps on the band. That would not damage the watch. (00:26:45)
Starring: Jennifer Aniston, Courteney Cox, Lisa Kudrow, Matthew Perry, David Schwimmer, Matt LeBlanc
The One Where Monica Gets a Roommate - S1-E1
Revealing mistake: As Monica is walking to her room and steps on Paul's watch, if you look carefully, you can see that she just steps on the band. That would not damage the watch. (00:26:45)

The One Where Ross Dates a Student - S6-E18
Visible crew/equipment: While Phoebe and Rachel are with Monica in the guestroom, Phoebe decides that Rachel should be the one to get the nice room, and right before Monica tells Rachel that she's the first guest at "Hotel Monica" we can see the end of the bedroom set wall, with the kitchen and living room visible.
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!
The One With The Prom Video - S2-E14
Trivia: The first day Courteney Cox donned the 'Fat Monica' make-up and costume, Matthew Perry walked right past her without recognizing who she was.
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