The One With Rachel's Big Kiss - S7-E20
Continuity mistake: When Melissa and Rachel are first reunited at the Central Perk, the way they hold their arms (crossed, hands on hips, etc.) changes with almost every shot. (00:00:45)
The One With Rachel's Big Kiss - S7-E20
Other mistake: Matthew Perry supposedly really likes wearing a tux that turns out to have been made for Diane Keaton. Except that Diane Keaton is 5'7" and Matthew Perry is 6 feet tall, and also considerably larger than the very slim Diane. He wouldn't even be able to put it on.
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.