The One Where They're Up All Night - S7-E12
Factual error: In the opening scene, Ross remarks on the stars the group is gazing at. In reality, there would be far too much light pollution in this part of New York City to be able to see any celestial bodies such as comets or stars.
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.