The One With All The Resolutions - S5-E11
Continuity mistake: Rachel mentions "the big thing," so Joey sticks his fingers in his ears to block her out. When Rachel pulls on his arms, the only things on the table are Joey's bottled drink, crayons, and coloring paper. In the next shot, we see Rachel's arms and she's not holding anything. It cuts to the next shot facing Joey, as he says, "Well then, it wouldn't be a secret, so yeah, that would be OK," and now there's a drinking glass on the table. Rachel removes it in the next shot.
The One With All The Resolutions - S5-E11
Continuity mistake: At the start, when the countdown hits 3-2-1, there's a closeup facing the TV, and in the TV's reflection the couch and armchair are both quite empty, but in previous and following shots both have people seated.
The One Where Rachel is Late - S8-E22
Continuity mistake: At the end, when Rachel and Ross leave for the hospital, Rachel grabs the keys. In the close-up, when Rachel asks, "Do you think we have time?" she holds the keys up in her left hand while Ross holds her empty right hand. It then cuts to a medium shot, and the keys are now in Rachel's right hand, the same hand Ross holds. (00:20:40)
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 Where Rachel is Late - S8-E22
Other mistake: Rachel's water breaks, but her skirt is completely dry.
The One Without The Ski Trip - S3-E17
Continuity mistake: At the start, just as Rachel is leaving Joey's apartment after mentioning the tickets she's got, Joey stops her and she turns round. Her hands are clasped together in front of her chest in one shot and by her side in the next. (00:04:00)
The One The Morning After - S3-E16
Continuity mistake: When Rachel asks, "When did Chloe leave?" the paper Ross is holding changes direction. Note the title of the paper: The New Yorker. (00:13:30)
The One With The Football - S3-E9
Continuity mistake: When they are outside for the first time, you see a shot from behind Monica, Joey, and Chandler. Monica then says, "Bring it in," and the shot changes. However, all 3 are now in different poses to what they were in before. (00:04:40)
The One With Phoebe's Ex-Partner - S3-E14
Continuity mistake: When Phoebe says you never run on a barge, her hand changes position on the table between consecutive shots. (00:06:50)
The One With All The Thanksgivings - S5-E8
Continuity mistake: The emergency room doctor's rubber gloves come and go between shots.
The One Where Ross And Rachel Take A Break - S3-E15
Visible crew/equipment: When Ross is on the phone to Rachel, near the start, just as he walks into the living area, a stage light is visible above him (only visible in fullscreen version). (00:06:10)
The One Where Monica and Richard Are Just Friends - S3-E13
Continuity mistake: When Monica is dabbing Richard's shirt, the way she is holding the cloth changes between shots. Note the amount over her fingers. (00:08:50)
The One With Ross's Inappropriate Song - S9-E7
Continuity mistake: When Chandler turns around and Joey watches the football game, the door to Chandler and Monica's room is open. However, when Joey sees the sex part of the tape in the next shot, as Chandler is about to turn around and the TV screen is visible, the door is shut. (00:14:29)
The One With Phoebe's Uterus - S4-E11
Revealing mistake: Phoebe already has a visible baby bump before she even agrees to get pregnant.
The One In Massapequa - S8-E18
Continuity mistake: When Monica shows Chandler the photo of Chi-Chi, as she says, "It's good stuff, huh," in the closeup Monica takes hold of the photo with her raised empty right hand, but in the next wide shot Monica is holding a Sharpie in her right hand and the photo is in her raised left hand. (00:02:25)
The One With The Stain - S8-E7
Continuity mistake: After Gunther responds to Ross with the biting "ezels" retort, it cuts to Monica and Brenda in the living room, and when Monica falls off her chair, the rug and blue pad underneath are suddenly rolled up at the corner, then the rug is properly flat. (00:11:15)
The One Where Ross Hugs Rachel - S6-E2
Other mistake: In this episode, Chandler says that "when it comes to sweets, [Joey] is surprisingly strict" and doesn't allow sweets around the house. Except that in Season Five, "The One With The Kips," we see that Joey leaves Rollo candies everywhere.
The One with the Monkey - S1-E10
Visible crew/equipment: When Joey the Elf deliberately jingles his elf shoes, Chandler's head may explode because of all the unspoken mocking jokes. It then cuts to a shot of Marcel, and the trainer's hand is visible in the bottom left corner of the screen. (00:07:25)
The One Where Rachel Tells... - S8-E3
Continuity mistake: After the opening credits, when Joey and Phoebe enter Joey's apartment, note the poster hanging on the wall and some items on top of the fridge, as well as a few of the magnets on the fridge, change, move, vanish, or suddenly appear in the following shots. Examples: the wall poster moves closer to the door; the Chunky B. Wear magnet at the side of the fridge moves to the freezer door; a takeout menu suddenly appears on the fridge door; the red star magnet vanishes; etc. (00:03:00)
The One With The Soap Opera Party - S9-E20
Other mistake: When Rachel walks in to her apartment right before the message of Jan Rogers, you can see the hole in the wall where the board usually is.
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.