The One With The Worst Best Man Ever - S4-E22
Continuity mistake: When Joey says that Ross needs to choose his best man before handing out of the ring, the camera cuts to Chandler. In the background you can see Joey getting up from the chair, but when it cuts back to Joey he is sitting down and then gets up in that shot. (00:02:01)
The One With The Worst Best Man Ever - S4-E22
Continuity mistake: In the scene where the veterinarian gives the ring back to Ross, there is a dog in the background that goes from standing up to lying down to standing again depending on the shot.
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.