The One With Joey's Dirty Day - S4-E14
Continuity mistake: In the scene where Mr Heston confronts Joey about using the shower, Mr Heston points at Joey from a seated position. As the shots switch Mr Heston's "pointing" hand also switches. (00:16:30)
The One with the Dozen Lasagnas - S1-E12
Visible crew/equipment: When Ross is on the balcony trying to comfort Rachel after she has split up with Paolo, we see a couple of shots from the balcony looking into Monica's apartment. If you look at the top of the screen you can see where the walls of the set end, and all the stage lights hanging from the ceiling of the studio. (00:18:35)
Continuity mistake: Just as Ross and Mona are about to dance, her arms are crossed. The camera cuts and they are on her hips. (00:16:05)
The One With Monica's Thunder - S7-E1
Continuity mistake: In the scene where Phoebe is telling Joey that his eyes "give him away" he is holding a sandwich; the camera angle cuts to behind him and the sandwich is on the table. (00:15:50)
The One With Princess Consuela - S10-E14
Continuity mistake: When Ross is trying to get Rachel's chair through the revolving doors, he has his right leg on the chair but it suddenly switches to him having his left leg on the chair. (00:15:25)
The One with the Dozen Lasagnas - S1-E12
Continuity mistake: When Ross is on about calling immigration, his hands move with no time to do so. (00:04:05)
The One Where Paul's The Man - S6-E22
Continuity mistake: When Rachel and Paul are on the couch at the cabin, the diet coke Rachel is holding switches (repeatedly throughout the scene) from her left hand to her right. (00:12:00)
The One Where Rachel Quits - S3-E10
Continuity mistake: At the start of the scene in which Ross breaks the little girl's leg, we can see that the stairs are just separated by railings. Later in that scene, when Ross and Chandler start walking up the stairs, there are walls on both sides of them. (00:01:40)
The One Where No One Proposes - S9-E1
Continuity mistake: In the one where Rachel is in the hospital after having Emma she is in the bed feeding Emma and Ross is sitting right beside of her. They are talking about their relationship and what should come. If you will watch each shot as they go to showing Rachel's face then Ross's you can see that Rachel's hair in one shot is in front of her shoulder and the next shot it is behind her shoulder. This happens several times. (00:20:55)
The One with Two Parts: Part 1 - S1-E16
Continuity mistake: At the start of the episode, Chandler is holding onto a menu with his right hand. In the next shot, it is gone. (00:00:10)
The One with the Stoned Guy - S1-E15
Continuity mistake: When the stoned guy says they should make macaroni and cheese, Monica holds out her hands. In the next shot, her hands are clasped together. (00:18:50)
The One with the Blackout - S1-E7
Revealing mistake: When Phoebe says something like, "Look, the last candle is about to go out", she starts counting down, saying "-47,-48..." and you can see Monica looking at the candle. But there is a light source behind her (it can't be a candle because she is looking at the last candle). (00:20:05)
The One with Two Parts: Part 1 - S1-E16
Visible crew/equipment: At the end of this episode when Rachel falls off the balcony, when we are looking out from Mr. Heckles' apartment, a boom mic is visible at the top of the screen just above him. (00:21:30)
The One with the Monkey - S1-E10
Continuity mistake: When Rachel says about her 6th date with Paulo, she is holding a decoration for the tree. In the next shot the decoration changes to a much larger object. Then, in the next shot, as she hands it to Monica, it has turned back to the original shape. (00:05:40)
The One with Two Parts: Part 1 - S1-E16
Continuity mistake: Just after Marcel switches the TV to "SAP," Joey comes back and tells Chandler about Ursula and how she gave him extra food. Chandler is holding a bottle of water, and before the camera cuts to Joey it is open, but when he is talking it is capped, and finally when Chandler says "Score," the bottle is open again. (00:07:05)
The One With the Baby on the Bus - S2-E6
Continuity mistake: In the scene where Chandler and Joey want to meet women with Ben, Chandler is holding the baby. A little after Chandler says, "Oh seriously you want him?" The position of Chandler's hands holding Ben changes between shots. (00:05:55)
The One Where No One's Ready - S3-E2
Continuity mistake: When Joey refuses to give Chandler the chair, Phoebe gets hummus on her dress, but in some scenes the stain is much smaller than others. (00:07:00 - 00:08:00)
The One With The Late Thanksgiving - S10-E8
Continuity mistake: When Rachel and Phoebe are talking in the Baby Beauty Contest and Rachel is holding Emma, Emma's head instantly changes position between shots several times, often going through 180 degrees. Clearly they couldn't get her to do the scene in one take and therefore the editing suffers. (00:07:05)
The One With The Routine - S6-E10
Visible crew/equipment: Just as Phoebe walks into the apartment with the duck and chicken (to help find Monica's hidden presents), a crew member's leg/foot can be seen at the bottom left side of the screen.
The One With The 'Cuffs - S4-E3
Continuity mistake: The handcuffs that Joanna uses on Chandler have a single metal bar connected via pivot to a double bar with the lock area. When Joanna first cuffs Chandler, the double bars on both sides face up at his wrists, but when Chandler's on the phone and when Rachel unlocks his right cuff, the handcuffs are going round his wrist the opposite way with the single bars facing up - quite impossible. Then the same thing occurs when Rachel handcuffs Chandler to the filing cabinet.
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.