The One In Barbados, part 2 - S9-E24
Revealing mistake: When Ross is talking to the weird obsessive scientist at the bar, just before the wife comes in, the wall behind the professor moves in shot to allow the cameraman room to swing around in the next shot. (00:40:20)
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.