The One Where Ross Moves In - S5-E7
Continuity mistake: When Chandler and Joey walk into the apartment to find boxes all over the place and also when Chandler talks about the air purifier, the door to the bathroom is open and the toilet is right beside the tub (odd that it's there in the first place), but when Joey tells Chandler he wants to play foosball and also when Joey's wearing the helmet, the prop toilet has vanished.
The One Where Ross Moves In - S5-E7
Continuity mistake: At the very start of the episode, when Joey's showing Chandler National Geographic, Chandler has his left hand up by his face in one shot, then when the angle changes they're both clasped in his lap. (00:00:05)
The One Where Ross Moves In - S5-E7
Continuity mistake: Chandler and Joey are on their couch thinking about what to do about Ross; when Chandler says "what are we gonna do?", he switches from crossed arms to hands folded in his lap. (00:15:45)
The One Where Ross Moves In - S5-E7
Continuity mistake: Rachel tells her neighbor Danny that she doesn't know anything about sailing, but later in the series, Rachel teaches Joey how to sail after he wins a boat at a silent auction. She says she learned sailing from her father while growing up.
The One Where Ross Moves In - S5-E7
Revealing mistake: When Joey and Chandler go in the bedroom to talk, Joey suggests they play a game of foosball. There is no ball when they start playing - quite noticeable when Joey " scores" a goal.
The One Where Ross Moves In - S5-E7
Revealing mistake: During Danny's party you can see Monica's apartment through the gap between the door and the wall, despite the party taking place on a lower floor. This is because they just reused the set for the hallway to save having to build a new set for the "lower floor."
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.