The One with the Monkey - S1-E10
Continuity mistake: Just before the opening credits, when Ross says, "That's 'pathet', which is Sanskrit for 'really cool way to live', " Marcel is sitting on Ross's left shoulder in the closeup, but in the next shot the monkey is suddenly sitting on his right shoulder, right before Ross walks off screen. (00:01:00)
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.