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 With The Birth Mother - S10-E9
Visible crew/equipment: When Joey's telling Phoebe about having been at a nice restaurant with his date and how she took fries off his plate, in the shots facing Joey in the restaurant we can see the end of the set wall, with a 2x4 framed plywood board and a heavy duty extension cord hanging there.
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 Dirty Girl - S4-E6
Question: When Joey asks why he can't give Kathy the pen, Chandler says, "Because she's not eleven. And it's not the seventh night of Chanukah." What is Chandler talking about when he says, "It's not the seventh night of Chanukah?"
Answer: Chanukah is a Jewish celebration that occurs around the same time as the Christian Christmas. The celebration lasts for eight days and it is customary to give a small gift on each of the eight days. There is no special gift required and the line about it being "the seventh night" of Chanukah is just Chandler's sarcasm.
Answer: Usually by the seventh night of Hanukkah you don't know what to get the other person, and you will probably resort to getting the other person something insignificant. Chandler is basically saying that the pen is a stupid gift.