Revealing mistake: When Phoebe and Ross hit heads while in the cab on the bridge, they only just tap heads. Not enough to make them scream in pain. (00:05:15)
Revealing mistake: When everyone leaves the apartment together for the last time, they show Chandler and Monica with the twins in the stroller, and you can tell they are dolls.
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.