The One with Two Parts: Part 2 - S1-E17
Revealing mistake: In the scene where Phoebe goes to see Ursula at Riff's, Ursula goes to get Phoebe her birthday present. When she sits down, you can easily tell, by looking at all the people there, that the shot has been sped up. (00:15:10)
The One with Two Parts: Part 2 - S1-E17
Revealing mistake: When the scene cuts from the apartment to Riff's, a man on the other side of the street walks past the Riff's sign and the sign is still visible through him, revealing it is just animated.
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.