The One With The Boob Job - S9-E16
Revealing mistake: When Phoebe and Mike walk into Central Perk and announce they're moving in together, Chandler, Monica, and Ross, who's wearing a sweatshirt and khakis, are all sitting on the couch, and after Phoebe's comment about loading up the Volvo and wanting to be a soccer mom, it's David Schwimmer's stand-in, wearing a blue button-down shirt and jeans, who is seated beside Chandler.
The One With The Boob Job - S9-E16
Continuity mistake: When Mike goes to leave Phoebe's apartment at the start, he puts his jacket over his arm twice. (00:02:10)
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.