The One Where Rachel's Sister Babysits - S10-E5
Continuity mistake: Rachel and her sister are in Rachel's room. When Rachel picks up Emma, Emma has nothing in her arms, but in the next shot when Rachel is holding her, she has a toy in her arms though she had no time to get one.
The One Where Rachel's Sister Babysits - S10-E5
Other mistake: Jennifer Aniston's tan mark from her wedding ring is especially visible in this episode. Most notably when she says "But Erin Brockovich had her own house."
The One Where Rachel's Sister Babysits - S10-E5
Continuity mistake: While everyone is booing Mike and Phoebe, the guy sitting behind Phoebe changes position in every shot.
The One Where Rachel's Sister Babysits - S10-E5
Continuity mistake: When Ross is looking at Emma in the coffee house he turns and looks at the guys on the couch and mouths the word "what." Then in the next shot he does it again.
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.