The One Where Everybody Finds Out - S5-E14
Continuity mistake: Monica and Chandler are deciding what to do about Phoebe's flirting; the shot cuts to her and Rachel, and Rachel's right hand is on her chin. In the next shot, it's her left hand on her chin. (00:12:00)
The One Where Everybody Finds Out - S5-E14
Continuity mistake: The amount of buttons undone on Pheobe's top by Joey changes after she says, "You didn't rip any buttons." The camera cuts to Joey then back, and it has changed. (00:18:00)
The One Where Everybody Finds Out - S5-E14
Continuity mistake: When Phoebe pours the white wine Chandler empties his glass to the last drop, but in following shots Chandler's glass has wine again, which then keeps vanishing and reappearing.
The One Where Everybody Finds Out - S5-E14
Continuity mistake: In the scene where Chandler turns on music when he and Phoebe are going to "have all the sex" you see Phoebe's arm begin to move in the right corner of the screen. When it cuts to her she has not yet begun dancing, and when he goes to turn the music on again it is a duplicate clip from the initial shot.
The One Where Everybody Finds Out - S5-E14
Continuity mistake: When Monica and Chandler come in to ask Joey about Pheobe knowing about their relationship he throws the Hugsy teddy behind him. In the next shot we see behind the chair and Hugsy is not there.
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.