Continuity mistake: When Joey and Chandler are discussing Joey's shoes, the length of the sleeve round Joey's neck changes depending on the shot. (00:11:20)
Continuity mistake: When Monica and Rachel are discussing Phoebe, Rachel's hands go from playing with the table to dry washing depending on the shot. (00:12:30)
The One with George Stephanopoulos - S1-E4
Continuity mistake: After Rachel speaks to her pregnant friend, they manage to swap places between shots. (00:05:50)
The One with the East German Laundry Detergent - S1-E5
Continuity mistake: At the start of the episode when Ross says, "Multiple orgasms", you can see Chandler's boot is not resting on his knee. In the next shot it has appeared from nowhere. (00:00:55)
The One With Joey's Big Break - S5-E22
Continuity mistake: When Rachel knocks over the cereal its position changes between shots. (00:04:15)
The One Where Everybody Finds Out - S5-E14
Visible crew/equipment: When Rachel stands up to get laundry for Monica, a stage light can be seen in the TV. (00:07:20)
The One With Ross's Wedding, part 1 - S4-E23
Visible crew/equipment: When Rachel gets back from shopping, all the stage lights can be seen in the tv. (00:11:50)
The One In Barbados, part 2 - S9-E24
Continuity mistake: At the end of the episode, when Joey walks into Rachel's hotel room, he is standing in the doorway. However, when Joey and Rachel kiss, there is enough space for the door to close. (00:47:05)
The One With The Rumor - S8-E9
Continuity mistake: When Rachel is offering Will the yams, the spoon that's in the dish is facing Rachel. When the shot changes to Joey lifting up the Turkey, the spoon is suddenly on the other side of the dish and facing towards Will. (00:11:05)
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)
The One Where They All Turn Thirty - S7-E14
Continuity mistake: When Monica is at the party and she is drunk, there is one point when she is sitting down on a chair with Chandler on her right. Chandler is holding her left hand (which has a snack in it), but when it cuts to a front view of Monica, suddenly both her hands are on her lap. (00:13:45)
The One With Joey's New Brain - S7-E15
Continuity mistake: When Joey is talking to the woman whose brain he receives, as the shot changes she is holding a glass, then a cigarette then a glass again, with no time to change objects, let alone anywhere to put the other. (00:06:05)
The One With The Jellyfish - S4-E1
Visible crew/equipment: When Ross storms out of the room and the rest of the friends are in the kitchen, you can see the stage lights at the top of the screen. (00:20:15)
The One With The Home Study - S10-E7
Continuity mistake: When Phoebe and Mike first donate the money to the children, the way the man is holding the cheque changes depending on the shot. (00:03:15)
The One with All the Poker - S1-E18
Continuity mistake: When Rachel finds out she has an interview she is holding the letter in her left hand. When the shot changes, the letter is in her right hand. (00:09:05)
Other mistake: When Chandler says "We just got married. Right there!" and points. We hear a small 'ding' and Monica asks "Is that your ring?" But when Chandler pointed, you can see that all of his fingers except his index finger are closed to his hand. So the ring couldn't have come off. (00:00:05)
The One with the Boobies - S1-E13
Continuity mistake: When Phoebe's boyfriend is talking about Chandler at the coffee shop, the cookie he is holding jumps from his left to his right hand in an instant between shots. (00:03:20)
The One with Mrs. Bing - S1-E11
Continuity mistake: In the final scene Rachel passes out copies of her draft for her book. She gives out all her copies and asks for their opinion. However, near the end of the scene after waving her arms around empty handed a copy of her script magically appears in her hands. (00:21:10 - 00:21:45)
The One With Phoebe's Uterus - S4-E11
Continuity mistake: When Phoebe is on the couch in the coffee house, talking to Monica and Rachel, the puppy moves and the way Phoebe is holding the puppy changes between shots. (00:17:00)
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.