The One After the Superbowl, part 2 - S2-E13
Revealing mistake: Rachel grabs a jar of sauce from the shelf and says, "You give me back my sweater or it's handbag marinara." You can tell from the sound it makes when it opens - no pop - that the jar had already been opened before she pulls the lid off.
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.