The One With Joey's Dirty Day - S4-E14
Continuity mistake: When Rachel is asking Ross if he will take Emily to the opera, the amount of biscuit in his hand changes throughout. (00:06:15)
The One With The Dirty Girl - S4-E6
Continuity mistake: At the beginning of the episode where they are watching Ross through the window, Phoebe is leaning forward and standing to the left of the pole next to Chandler. When the shot changes to Ross coming through the door, she is suddenly standing up straight and is now behind Joey and Chandler. (00:00:10)
The One With Ross's Wedding, part 1 - S4-E23
Continuity mistake: Rachel is cutting flowers and she and Phoebe are discussing if she loves Ross. Phoebe's necklace switches sides every time the shot goes back to her. (00:07:35)
The One With Joey's New Girlfriend - S4-E5
Continuity mistake: In the bar, Kathy is trying to set Chandler up with one of her friends. When the camera is facing Chandler, we see Kathy's hand rubbing Joey's back, but when it cuts to a front shot of Joey and Kathy, her arm is down in front of her. Then without having had enough time to move, her elbow is up on the table and she is holding hands with Joey. (00:16:10)
The One With Ross's Wedding, part 2 - S4-E24
Continuity mistake: In the scene where Monica and Ross discuss how Emily has decided to call off the wedding after Monica suggests they postpone the wedding, Monica is wearing a black watch on her left wrist. This disappears and re-appears a number of times throughout the scene. (00:15:00)
The One With All The Haste - S4-E19
Continuity mistake: Phoebe comes in wearing Santa pants. When Joey and Chandler point this out, Chandler's legs are crossed or uncrossed, depending on the camera angle. (00:04:10)
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.
The One With Joey's New Girlfriend - S4-E5
Continuity mistake: During the scene where Chandler is playing hide and seek with the duck and the chick, the duck moves from facing the opposite way to him, facing the same way, and then the opposite way as Joey walks in. (00:13:35)
The One With Joey's New Girlfriend - S4-E5
Continuity mistake: When Chandler is chasing Cathy, the mustard on his shirt changes place, size and darkness throughout the scene. (00:11:45)
The One With Ross's Wedding, part 2 - S4-E24
Continuity mistake: When Felicity finds Joey outside the wedding venue she closes the door. But when they start kissing, the door is completely open. (00:35:50)
The One With Joey's New Girlfriend - S4-E5
Continuity mistake: Chandler is chasing Kathy down the street trying to get her attention. During a close-up of him running, just before the camera cuts away we see a guy's face on the left; when it cuts back to Chandler he runs for a while and then encounters the same guy (the dog-walker). Also, when he finally lands in front of Kathy, there is a bike rider coming towards them, but as the camera cuts back and forth between the two, the rider never passes them. (00:11:30)
The One With The Embryos - S4-E12
Continuity mistake: In the last scene, Chandler is sitting on the couch in the new apartment, and the rooster is sitting on the back of the couch, with his tail facing the window. When Joey comes running out, the shot of the rooster shows his face toward the window, and then in the next shot he has returned to his tail facing the window. (00:20:45)
The One With All The Wedding Dresses - S4-E20
Continuity mistake: When Chandler is telling Monica and Phoebe about the girl who picked him over Joey, look behind him and you will see Gunther nodding to a lady in a denim jacket. In the next shot, the woman walks up to the counter again, and again Gunther is nodding at her. (00:17:00)
The One With Ross's Wedding, part 1 - S4-E23
Continuity mistake: When Monica, Ross and Emily are going to see the church, Emily's hands move between shots with no time to do so. (00:05:30)
The One Where Chandler Crosses The Line - S4-E7
Continuity mistake: When Chandler is showing Joey all of the new stuff in the apartment, the cuff on his right sleeve keeps going from messy to neat between shots. (00:13:45)
The One With Rachel's New Dress - S4-E18
Continuity mistake: In The One With Rachel's New Dress, she's in her underwear on the sofa when Joshua comes in. In wide shots focusing on Joshua she's quite laid back, but in close shots of her she's sitting more upright. Worse than that, her legs are crossed, but uncross in the shot of his parents coming in, only to re-cross in the next shot when she gasps. (00:14:35)
The One Where Chandler Crosses The Line - S4-E7
Continuity mistake: When Ross and Phoebe are talking in Central Perk, Phoebe's position changes. (00:14:45)
The One With Ross's Wedding, part 2 - S4-E24
Continuity mistake: As Ross says, "This is our wedding day," in a close-up, Elliott Gould is holding a bit of paper in his left hand (glasses in his other hand, out of shot). In the wide shot, the paper is in his right hand, then jumps back to his left when the angle changes. (00:36:25)
The One Where Chandler Crosses The Line - S4-E7
Continuity mistake: When Cathy and Chandler are talking while sitting inside the boat, the silver object on the corner of the counter behind Chandler disappears between shots. (00:08:20)
The One With Rachel's New Dress - S4-E18
Continuity mistake: When Rachel shoos the birds into the hall, the chicken turns around and heads away from the guys' door, however when Joey opens the door the chicken is right outside the door. (00:10:15)
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.