The One With The Proposal, part 2 - S6-E25
Visible crew/equipment: After Joey fibs and tells Chandler that Monica went to her parents, when Chandler and Joey walk into the apartment, in the shot facing the hallway we can see some of the camera equipment past the doorway, and in the next wideshot of the apartment as the camera pans to Monica the overhead studio lighting rigs can be seen at the top of the screen.
The One With The Proposal, part 2 - S6-E25
Continuity mistake: When Monica is in Richard's apartment she gets all worked up, so Richard gives her a hug to calm her down. She goes into the hug with her arms curled up in front of her face, but when the camera angle changes, they are at Richard's back. (00:13:50)
The One With The Proposal, part 2 - S6-E25
Visible crew/equipment: When Chandler walks in and sees Monica, the boom mic is visible above Monica's head. (00:17:45)
The One With The Proposal, part 2 - S6-E25
Audio problem: In the pizza place, after Chandler tells Monica to "put down [her] copy of The Rules..." you hear Monica say, "You know what. I gotta go" when her lips aren't moving. (00:04:26)
The One With The Proposal, part 2 - S6-E25
Continuity mistake: At the end when Chandler returns to his apartment, from the outside he opens the door with his right hand. Then it cuts to an inside view and he's opening it with his left hand on the handle. (00:17:40)
The One With The Proposal, part 2 - S6-E25
Continuity mistake: Right after Monica says, "You wanted it to be a surprise", the camera pans to Joey who shuts the door firmly, however in the next shot you can see the door gliding back open and then shutting again. (00:18:40)
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.