The One with the Thumb - S1-E3
Continuity mistake: When Chandler smokes the cigarette at the start of the episode, keep an eye on Joey's arms. First he moves his arms between his legs, then his left arm moves onto his knee between shots. (00:03:45)
The One with the Thumb - S1-E3
Continuity mistake: When Ross said his parents sent their dog away, Chandler is holding onto his right foot with his hand. In the next shot, the foot is gone and his arm is in a different position. (00:01:00)
The One With Ross's Tan - S10-E3
Continuity mistake: Ross' tan keeps changing shades. It gets lighter then darker, then lighter again. (00:12:40)
The One With The Racecar Bed - S3-E7
Continuity mistake: In the scene where Joey's teaching soap opera for actors, his name changes from the 'a' being on the line to being above it. The line itself also reaches the far right of the board in some shots, and is short of it in others. (00:06:00)
The One with George Stephanopoulos - S1-E4
Continuity mistake: When Phoebe talks about the cave in in the mine, Ross has his hands in his pocket behind her. In the next shot his hand is on the sofa. (00:04:10)
The One with the Blackout - S1-E7
Continuity mistake: During the Monopoly game (deleted scene), right after Monica says, "we can bring my wheelbarrow, to carry your teeth," we see Ross reading the game manual at the end of the shot. At the beginning of the very next shot, he is suddenly rolling the dice with no manual in his hands. (00:17:00)
The One with the Birth - S1-E23
Continuity mistake: In the scene when Carol is giving birth, there is a shot when Carol shouts "Where were you?" Her trousers are visible under the sheet. (00:16:25)
The One With The Apothecary Table - S6-E11
Continuity mistake: In the scene where Monica and Janine are in the hall, Janine goes to run and lifts up her arms, but the next shot shows her lifting up her arms again. (00:19:50)
The One Where Phoebe Runs - S6-E7
Continuity mistake: When Phoebe is waiting for Rachel behind the tree a man wearing grey runs past (left to right). Then when Phoebe jumps out and is talking to Rachel about her fake sore leg, the man in grey runs past again. (00:08:00)
The One Where Phoebe Runs - S6-E7
Continuity mistake: In the episode 'TOW Phoebe Runs' when Rachel runs into the horse at the end there is a shot of the horse looking round at her, when the shot changes angle to show Rachel picking herself up you can see that the horse is still looking straight ahead. (00:18:35)
The One With Rachel's Date - S8-E5
Continuity mistake: When Rachel asks what Kash said, her arms are outstretched, the ext shot they are together and when the camera cuts again and they have moved yet another time. (00:06:55)
The One With The Soap Opera Party - S9-E20
Continuity mistake: When Ross is telling Rachel and Phoebe about Charlie's past boyfriends, he refills two wine glasses. Then in the next shot, he puts his hands to his cheeks in shock because he just remembered something, without putting the wine bottle on the table. It just disappears from his hands. (00:16:05)
The One Where Rachel Tells... - S8-E3
Continuity mistake: When the airline representative states the first class seats are gone, her head leans away from the camera. The shot changes and it's leaning to the camera. The camera is in the same place. (00:07:10)
The One Where Rachel Tells... - S8-E3
Continuity mistake: When Rachel and Ross are at his apartment, Rachel is holding some magazines. The way she holds them changes depending on the shot. (00:09:35)
The One With The Red Sweater - S8-E2
Continuity mistake: When Chandler mentions about the disposable cameras to Monica to cheer her up, Joeys hands are together. Next shot they are apart. (00:00:50)
The One with the Blackout - S1-E7
Continuity mistake: After the light comes back on and Ross is holding his heart after he sees Rachel kissing Paulo, in the next shot when Joey starts speaking to him his hand is by his side. (00:20:40)
The One Where Chandler Takes a Bath - S8-E13
Continuity mistake: When Chandler is in the bath with Monica at the end a splodge of bubbles appears on his face between shots. (00:20:40)
The One Where Ross Can't Flirt - S5-E19
Continuity mistake: When Phoebe asks Rachel for the earrings she borrowed, Chandler is visible in the background, standing up with one leg on the table and talking to Joey. When the camera angle switches, Chandler is sitting down. (00:05:00)
The One With The Triplets - S5-E3
Continuity mistake: When they enter the hospital, Phoebe says, "Someone should call Frank and Alice, and my mom," and Ross searches his pockets(presumably for his cell phone). But in the next shot, his hands are not in his pockets. (00:00:05)
The One Where the Monkey Gets Away - S1-E19
Continuity mistake: When Ross comes back to Monica and Rachel's apartment and Rachel tells him she has lost Marcel, he takes off his overcoat and lays it over the back of the sofa and then sits by it. As the scene continues, the sofa is still visible in some shots and the black coat is crumpled up on the sofa cushion. (00:11:25)
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.