The One With The Home Study - S10-E7
Audio problem: While the adoption social worker, Laura, is interviewing Monica and Chandler, Monica mentions to her that she's welcome to look under any of the furniture where she won't find cigarettes or porn, and then it cuts to Laura's close-up as we hear her say, "Well, actually before we look around," but her mouth is not saying those words.
The One Where Ross Is Fine - S10-E2
Audio problem: When Ross is drunk and talking to Rachel, Joey and Charlie on the couch in his apartment; he begins to say "Where do you think we can go?" but his mouth does not move. (00:12:10)
The One With The Birth Mother - S10-E9
Audio problem: When Monica and Chandler are talking about telling Erica, when Monica says "Technicality!" It doesn't match with her lips. (00:15:30)
The One Where Ross Is Fine - S10-E2
Audio problem: When Phoebe and Frank Jr. are talking about giving one of the triplets away, Frank makes a very loud noise but his mouth doesn't move. Making a moan this loud requires you to open your mouth. (00:12:50)
The One With The Cake - S10-E4
Audio problem: When Chandler says, "Ironically, that was the Maxim thing", you can tell it's an overdub. The volume and the background noise is different, and we don't see a shot of him speaking these words. (00:11:20)
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.