The One With Rachel's Going Away Party - S10-E16
Continuity mistake: The level of beer in Ross' bottle goes up and down between shots as he's talking to Joey at the kitchen table. (00:12:00)
The One With Rachel's Going Away Party - S10-E16
Continuity mistake: When Ross fears that Rachel is going to talk with him, the volume and the color of his beer changes between shots.
The One With Rachel's Going Away Party - S10-E16
Plot hole: In this episode Chandler finds furry black handcuffs, and everybody says that they have never seen them before. How is that possible considering all the people that have moved in and out of that room? How come nobody noticed them when the room and closet were painted green after being pink?
The One With Rachel's Going Away Party - S10-E16
Revealing mistake: The twins are dolls. They don't move, and the way they are being carried indicates they don't weigh much.
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.