The One With Ross's Sandwich - S5-E9
Continuity mistake: In Central Perk, when Phoebe writes a note for Ross to put on his sandwich so no one eats it, she mentions that she stabbed a cop, and has a green mug in her hand. It cuts to Monica then back to Phoebe and the mug is out of her hand and she's yelling, "He stabbed me first." When it cuts back to the wide shot the mug is back in Phoebe's hand. (00:05:40)
The One With The Hypnosis Tape - S3-E18
Visible crew/equipment: When Monica leaves the apartment and everyone is watching you can see the stage lights on the ceiling in shots from Pete's viewpoint. (00:18:10)
The One Where Rachel Has a Baby, part 1 - S8-E23
Continuity mistake: When the couple who has bad pet names for each other is sharing the maternity room with Rachel, the man is unpacking a suitcase that he closes, but in the next shot its open and he flips the top closed again. (00:09:10)
The One Where No One Proposes - S9-E1
Continuity mistake: In the scene where Rachel believes Joey has proposed to her at the hospital after having Emma. Ross is talking to Rachel about her saying yes to Joey's proposal. The nurse brings Emma in to breast feed. When the nurse leaves, she closes the door. However, when they shoot back to Joey a couple of times before he leaves the room, the door is open. (00:19:45)
The One Where Estelle Dies - S10-E15
Continuity mistake: Near the end when Chandler is talking to Janice at one point his hands go from pointing straight ahead to his pockets. (00:17:20)
The One with the Blackout - S1-E7
Continuity mistake: In the scene where Joey is standing with candelight over Ross' neck, the candle is full length, but in the next shot it's only half length. (00:13:00)
Continuity mistake: In the scene just before Joey goes into the shower, when he is thanking the director for the opportunity, the camera shot goes back and forth between Joey and the director several times. During this series of shots, the part in Joey's hair switches sides nearly every time. It appears that the entire shot is reversed - in the closeup, the shower curtain is on his left and the shadow to his right is sharply focused. In the lower (long) shot, the curtain is to his right, and the shadow on his left is more focused. (00:13:05)
The One With Rachel's Inadvertent Kiss - S5-E17
Revealing mistake: When Ross is doing his "Watching TV" bit, there is a brief shot of the blank screen that he is sitting on the sofa pretending to watch. In the screen's reflection, it is clear that the sofa is empty. (00:03:55)
The One Where Phoebe Hates PBS - S5-E4
Continuity mistake: Monica comes to get her pan back from Chandler, and when she opens the door, the magna doodle pen is hanging down. The shot changes and it is attached to the board; it changes again and it is hanging once more. (00:12:00)
The One Where Rachel Finds Out - S1-E24
Visible crew/equipment: When Rachel is on the balcony with her date, after she kisses Ross and says, "Wow!", the shot changes back to her date. While he is saying, "Exactly" you can see a camera man on the left side of the screen. Then when the shot changes to show Rachel and her date you can see him move out of the shot. (00:19:45)
The One With Phoebe's Husband - S2-E4
Continuity mistake: In the scene where Rachel is telling Ross it is sexy for a man to wait before having sex, the cups behind Ross change color: red to green to red and back again. (00:08:10)
The One With Ross's Sandwich - S5-E9
Continuity mistake: In the scene where Joey has just brought home his date, he jiggles the door handle, which makes Monica and Chandler run off into Chandler's room. Chandler kicks a circle cushion off the mat, but when the camera cuts back, it's on the mat again, and then off the mat again when Rachel walks in. A red cushion also moves. (00:09:10)
The One With Rachel's Going Away Party - S10-E16
Continuity mistake: The level of beer in Ross' bottle goes up and down between shots as he's talking to Joey at the kitchen table. (00:12:00)
The One With Ross's Denial - S6-E3
Continuity mistake: When Monica and Chandler are eating, Monica's plate holds a sandwich and chips. When Joey comes in, Monica's plate contains no chips, and when he leaves the chips are back again. (00:04:00)
The One With Ross' New Girlfriend - S2-E1
Continuity mistake: At the beginning when Rachel falls and starts bleeding, you can see that the wound is somewhere up above her forehead because the blood is running from the wound up there. However, throughout the rest of the episode she has a bandaid going horizontally across her forehead, nowhere near where the apparent wound was. (00:01:20)
The One With The Cake - S10-E4
Visible crew/equipment: When Monica is talking to Chandler about waking Emma up, as she walks over to him, crew lights are visible in the top left corner of the screen. (00:05:35)
The One With Phoebe's Cookies - S7-E3
Continuity mistake: While trying out different recipes, at one point Monica's holding a cookie in her hand. It's a whole cookie, but when the angle changes, suddenly a chunk is missing. (00:13:00)
The One With All The Thanksgivings - S5-E8
Continuity mistake: In the scene where they take Chandler to the hospital, Monica hands the bag of ice containing Chandler's toe to the doctor. The doctor is wearing gloves in one shot then no gloves in the next. This goes back and forth three or four times. (00:07:45)
The One With Joey's Award - S7-E18
Continuity mistake: When Joey sets down the award he sets it at a straight angle to the counter edge, but when Joey catches Rachel about to pick it up it is angled and not straight. (00:13:45)
The One Where Estelle Dies - S10-E15
Revealing mistake: When Chandler and Monica are at the door of the house you can see that it is a fake background behind Chandler. (00:09:35)
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.