The One with the Thumb - S1-E3
Continuity mistake: When Monica ask's Ross if she looks OK, her hand is raised or by her side depending on the shot. (00:09:45)
The One with the Sonogram at the End - S1-E2
Continuity mistake: When Barry is explaining how he is an orthodontist, he is holding a dental tool up. Camera cuts and now it's lowered. (00:19:30)
The One with the Sonogram at the End - S1-E2
Visible crew/equipment: Ross stands up to tell his parents about what has happened with Carol. When he does he walks round the table and stands under a very visible boom microphone. (00:12:05)
The One with the Sonogram at the End - S1-E2
Continuity mistake: When Monica has her elbow on Ross' hand, her fist is under her chin. The camera changes to a different angle and now her hand is no longer under her chin and to the side of her jaw. (00:12:00)
The One with the Sonogram at the End - S1-E2
Continuity mistake: When Monica is asking if Ross has any stories to tell, she is looking at Ross and grinning. When the camera cuts, her expression has changed massively. (00:12:00)
The One with the Boobies - S1-E13
Continuity mistake: When Joey's dad walks in and sees Ronnie, he shuts the door. However, as they go to leave to a hotel the door is open and Joey shuts it. (00:11:50)
The One with Two Parts: Part 1 - S1-E16
Continuity mistake: When Chandler is saying how cute Nina is, Rachel has her hand in the bag of Chinese food. When the camera cuts, the hand is gone. (00:05:55)
The One with the Candy Hearts - S1-E14
Continuity mistake: When Chandler are Joey are talking in the restaurant and Chandler says "Me and you 'we'" his right hand is raised, camera changes and it's lowered. (00:07:30)
The One Where Nana Dies Twice - S1-E8
Continuity mistake: At the funeral, Judy says to Monica that Nana would have hated this [the funeral]. When she does, Monica's arm changes position between shots. (00:18:50)
Trivia: In one of the intros used in season 1 on TV you can see a clip of Ross, Chandler and Joey dancing in Ross' apartment. That clip is not in any aired episode. It's in the extended version of "The One with All the Poker".
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.