The One Where Rachel Tells... - S8-E3
Continuity mistake: At the start, before Monica and Chandler leave for their honeymoon, note the chairs around the kitchen table. When Rachel walks in to tell the couple to have a good honeymoon, the chair in front of the fridge and the chair in front of the stove have switched places.
The One In Massapequa - S8-E18
Revealing mistake: During Jack and Judy Geller's party, there are many shots where Elliott Gould's (Jack Geller) stand-in is perfectly visible. First, when Catherine walks over to offer congratulations to Rachel and Ross. Second, when Ross looks directly at his "dad" to ask if they have to pretend they're married. Then, numerous wide shots of "stand-in Jack" and Judy seated together in the background, such as after Rachel mentions the ring being the size of her fist, and during the toasts, etc. (00:06:00)
The One With Monica and Chandler's Wedding, Part 2 - S7-E24
Revealing mistake: At the point in the ceremony where Joey forgets the rings, the camera pans out and you see Chandler step sidewards to get the ring from Ross, and Monica step sidewards to get the ring from Rachel and Phoebe. As Monica moves, her dress slides across and one of her feet is revealed; it has what looks like a brown boot or heavy brown shoe on it. (00:20:05)
The One With The Bullies - S2-E21
Continuity mistake: When Phoebe calls by the house with the dog, a woman with a handbag is visible in the background, going into her house. About ten seconds later we see the same woman with the handbag walk towards her house again. (00:15:40)
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 Phoebe's Ex-Partner - S3-E14
Visible crew/equipment: Just after Chandler says "I'm the smallest person in the world", and Joey comes out of his room, check the top left of the screen - you can see a boom mic track Matt LeBlanc across the set, and the bottom of some stage lights in the middle. (00:14:40)
The One With the Baby on the Bus - S2-E6
Continuity mistake: When Stephanie is singing "Angel in the Morning", there is a person in overalls sitting behind the green pole. The shot changes to Phoebe outside and you can still see the person; it changes back to Stephanie and the person is now sitting on the next table. (00:16:00)
The One With Phoebe's Ex-Partner - S3-E14
Continuity mistake: When Monica comes in and asks Chandler if she can borrow some milk, Chandler is reading a paper that is flat on the bar. In the next shot he is holding it folded, then he lays it on the counter, and suddenly it is in his hands again. (00:14:20)
The One The Morning After - S3-E16
Visible crew/equipment: When Phoebe says they've been quite for a long time, a boom mic is visible above her (only visible in fullscreen version). (00:21:40)
The One Where No One's Ready - S3-E2
Visible crew/equipment: After Rachel agrees that it will indeed take her 6 or 7 minutes to get dressed, with the caveat, "Once I figure out what I'm wearing," it cuts to Joey offering Ross the "glass of fat" as he stands behind the couch. In the next shot of Ross, the reflections on the TV screen show Joey is still sitting on the couch, and the camera and camera operator (wearing a dark shirt and raises an arm) behind the sofa. (00:01:15)
The One Where No One's Ready - S3-E2
Visible crew/equipment: When Ross tells Chandler and Joey, "I'd feel a whole lot better if you got dressed now," the reflections on the TV screen show there's a crew member at the side of the sofa wearing a white T-shirt, as well as the camera and camera operator behind the sofa. (00:00:30)
The One With The Bullies - S2-E21
Visible crew/equipment: When Chandler falls behind the couch, the mat he lands on is visible.
The One Where Old Yeller Dies - S2-E20
Continuity mistake: A woman and a man in a blue shirt exit the coffee shop twice.
The One With The Bullies - S2-E21
Visible crew/equipment: While driving the cab, Phoebe is wearing a blue shirt. When the car stops suddenly, a crew member wearing a white shirt is visible driving the car.
The One With All The Jealousy - S3-E12
Continuity mistake: At the start, Ross says, "Botanists are such geeks." When he does, his left hand goes from in his lap to on the table instantly. (00:00:35)
The One Where Chandler Can't Remember Which Sister - S3-E11
Continuity mistake: When Joey is saying about ticking his Grandma off, he has his hands up in front of him. When the shot changes, his hands are lowered. (00:13:20)
The One With The Giant Poking Device - S3-E8
Continuity mistake: When Ben says Monica's name she's holding the Minute Maid juice box, but in Ben's closeup the juice box is on the table, though it's back in Monica's hand in the next shot. (00:08:50)
The One With The Giant Poking Device - S3-E8
Continuity mistake: At the start, Phoebe snaps at Rachel that she will go to the dentist. When she says this, her grip on the apple in her hand changes between shots. (00:02:30)
The One With Chandler In A Box - S4-E8
Visible crew/equipment: After Joey tells Chandler he's going to his folks and might stay there for a few days while looking for an apartment, just as Joey retrieves his keys from the shelf, a crew member wearing a black shirt and white pants is visible on the left side of the screen. (00:08:40)
The One Without The Ski Trip - S3-E17
Continuity mistake: When Monica and Phoebe enter the coffee house and talk to Chandler and Ross, the extra sitting at the coffee bar (black shirt, blue jeans) is not wearing a belt, then when they pan again she is, and back she is not. (00:00:12 - 00:00:54)
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.