The One With Monica and Chandler's Wedding, Part 1 - S7-E23
Revealing mistake: When Monica leaves the apartment and Chandler stays listening to the answering machine, check the wall beside the door. It changes the angle, as if somebody moved it (probably to hide the hall).
The One With Monica and Chandler's Wedding, Part 1 - S7-E23
Revealing mistake: When Monica leaves her apartment in the scene where she thanks Chandler for staying calm since they got engaged you can see the wall on the left side of the door moving as the door closes.
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.