The One With The Flashback - S3-E6
Other mistake: When Phoebe's holding the box of Nabisco Nilla wafers, the Nabisco logo has stickers and paint hiding it and the Nilla is also partially covered so it reads "Villa" wafers.

Starring: Jennifer Aniston, Courteney Cox, Lisa Kudrow, Matthew Perry, David Schwimmer, Matt LeBlanc
The One With The Flashback - S3-E6
Other mistake: When Phoebe's holding the box of Nabisco Nilla wafers, the Nabisco logo has stickers and paint hiding it and the Nilla is also partially covered so it reads "Villa" wafers.

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.
Trivia: The Magna-doodle on the door in the boys apartment has a different picture on it every episode. There is usually some tenuous connection between the picture and the plot of the episode.
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