The One Where Underdog Gets Away - S1-E9
Continuity mistake: When Rachel turns up with her tickets, Chandler has his hand on his hip. As Rachel walks off however, his arm is behind her. (00:12:00)
Continuity mistake: When Ross is reading the book he shows it to Phoebe who passes it back. Then without turning the pages he passes it to Chandler but the pages are different now. (00:07:20)
Continuity mistake: When Joey asks how the play was, Monica's grip on her program changes between shots. Note the gap between her hands. (00:04:20)
The One with Two Parts: Part 1 - S1-E16
Continuity mistake: Towards the end, as Joey is leaving Central Perk, Rachel's posture goes from leaving over the back of the sofa with her hands clasped to standing up with her hands on the back of the sofa. (00:18:15)
The One with Two Parts: Part 1 - S1-E16
Continuity mistake: When Rachel says about the name being the same in Spanish and English, the way she holds her chopsticks changes between angles. (00:06:10)
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 Stoned Guy - S1-E15
Continuity mistake: When Monica asks for the gummy bears near the end, she has her hands by her side. As the camera changes, a hand is now visible raised that should have been visible before. (00:20:00)
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 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 Mrs. Bing - S1-E11
Continuity mistake: When Coma Guy asks who Monica and Phoebe who they are, he has his hands lowered on the bed in one shot and raised outstretched in the next. (00:18:30)
The One with Mrs. Bing - S1-E11
Continuity mistake: The way Joey holds the picture of his mum changes between shots. Note his fingers. (00:15:50)
The One Where Nana Dies Twice - S1-E8
Continuity mistake: When Rach asks who the naked picture is of, the way she holds it changes between shots. (00:21:10)
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)
The One Where Nana Dies Twice - S1-E8
Continuity mistake: When Monica is talking to her dad at the table, the way he holds the magazine changes between shots. (00:08:50)
Continuity mistake: At the very end when Monica is in bed, her grip on the bed clothes changes between shots. (00:23:10)
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 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 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 That Could Have Been, part 2 - S6-E16
Continuity mistake: When Chandler goes into Monica's apartment after he beeps Dr Roger for a "ruptured spleen" his hair is sticking up through the front of his cap. After he tells Monica "You should be with me", the shot goes to Monica and when it goes back to him, his hair is neatly tucked into the cap.
The One With Ross's Grant - S10-E6
Continuity mistake: After Joey puts the video in the player, it's clear the video doesn't go all the way in, and even starts to eject itself. The shot cuts, and without him making any moves to replace the tape, it plays fine.
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.