The One With The Girl Who Hits Joey - S5-E15
Other mistake: When Ross and Phoebe are eating cheese puffs and party mix out of the bags, Phoebe's bag reads "Heez-I" instead of Cheez-It, and the brand "Sunshine" has been covered.
The One With The Girl Who Hits Joey - S5-E15
Continuity mistake: When Ross tells Chandler and Rachel that he's running out to buy name tags for his party, in the next shot the water bottle beside Rachel does not have its cap on, but it cuts to Rachel's closeup and the cap is on. Other differences can be seen as well, such as Rachel's hair and an extra utensil.
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.