The One After Ross Says Rachel - S5-E1
Audio problem: Monica tells Ross that he should keep looking for Emily, and Chandler says "Yeah, for about 30 minutes." But the audio doesn't match up with his lip movements, indicating this line was dubbed. (00:10:26)
The One With The Ball - S5-E21
Audio problem: When Rachel is trying to sell the cat and someone looks the hiss is a clear overdub. (00:16:20)
The One With Joey's Big Break - S5-E22
Audio problem: At the very end when Ross hits Rachel's stomach twice the sound doesn't match his actions. (00:21:00)
Audio problem: When Rachel and Ross are in Central Perk, Ross says "Well there's this one thing." Then it cuts to a shot of the side of his head where he says "Okay" and it doesn't match his lips.
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.