The One With the Holiday Armadillo - S7-E10
Continuity mistake: During this episode, Chandler is having major problems slipping people dollar bills. He asks Monica if Richard (her ex) use to slip people notes. But in Series 2 there was an episode where Chandler and Joey were doing impressions of Richard; they impersonate him by slipping money to each other with no problems. So not only has Chandler forgotten that Richard used to slip money, he's also lost the ability to slip money himself. (00:05:05)
The One With the Holiday Armadillo - S7-E10
Continuity mistake: When Ross is on the couch telling Ben about Chanukah, Ben's hair varies from shot to shot. In the first shot it's flat and in the next shot it's spiked up at the front. His hair stays spiked up for a few shots and then returns to flat. (00:06:00)
The One With the Holiday Armadillo - S7-E10
Continuity mistake: In the scene where Ross takes a liquorice from the skull, in the first close up shot he is holding it in his hand, but then immediately after this when they zoom out, he is chewing a piece. This is impossible because he would have had no time to bite it. (00:00:50)
The One With the Holiday Armadillo - S7-E10
Revealing mistake: When Ben sits on the sofa to listen to Ross talk about the festival of lights, you can see Monica move Ben to the other side of the sofa despite there being space for her to sit. This is so they sit in the correct order as she would mask Ben to the cameras.
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.