The One With The Donor - S9-E22
Continuity mistake: When Phoebe enters the coffee house to get Joey's advice on her outfit for Mike's sister's party, it cuts to Joey saying "You look stop-eating hot", and Phoebe's left hand can be seen making a nervous up-down motion in response. But when it cuts back to her, it is her right hand making the motion and her purse is now in her left hand.
The One With Rachel's Date - S8-E5
Visible crew/equipment: When Rachel and Cash leave Joey's apartment, in the top-left corner a stage light is visible. (00:12:05)
The One With The Cake - S10-E4
Visible crew/equipment: At Emma's birthday party, Joey and Phoebe are discussing the performances they are doing for Emma. The balloons placed behind them reflect the stage and spot lights. This happens throughout the episode in most of the balloons.
The One With Monica and Chandler's Wedding, Part 1 - S7-E23
Revealing mistake: When Monica leaves the apartment and Chandler stays listening to the answering machine, check the wall beside the door. It changes the angle, as if somebody moved it (probably to hide the hall).
The One Where Monica Sings - S9-E13
Visible crew/equipment: When Gavin goes to the door to bring Rachel soup you can see a camera man's microphone in the top of the screen. (00:10:50)
The One With The Birthing Video - S8-E15
Revealing mistake: When Monica gets out of her room to show Chandler her sexy clothing, as she opens the room's door, the buildings from across the street move. The scenario was a curtain and the wind that the door made when she opened it on the set made those curtains with the buildings printed in move. (00:06:25)
The One With Monica and Chandler's Wedding, Part 1 - S7-E23
Revealing mistake: When Monica leaves her apartment in the scene where she thanks Chandler for staying calm since they got engaged you can see the wall on the left side of the door moving as the door closes.
The One With The Bullies - S2-E21
Continuity mistake: When the fight scene starts, one of the bullies takes off his watch and puts it into his right jacket pocket. When they put all of their jewellery into his cap, he gets his watch out of his right jeans pocket.
The One Where Chandler Doesn't Like Dogs - S7-E8
Continuity mistake: When Rachel and Tag are on the balcony and Tag says, "What did Joey say? I like you back?" he is standing on the balcony with one hand in his pocket and the other in front of him. As the camera angle changes, Tag is now in a different position, with both his hands behind him, leaning against the balcony before he moves forward.
The One With Princess Consuela - S10-E14
Visible crew/equipment: At the very start when Monica mentions swinging, all the stage lights are visible in her glass of red wine. (00:00:20)
The One With Ross's Wedding, part 1 - S4-E23
Other mistake: When Phoebe tells Rachel that she thinks she can help her get over Ross, she starts by saying that she needs some photos of Ross. Rachel immediately starts walking towards the kitchen, walking right past the red box of photos on the table beside Phoebe (which she will pick up on her way back and then hand to Phoebe), before Phoebe even mentions the chocolate (and then glass of water) she also needs. It's obvious that Rachel is headed for the kitchen before she knows she needs to go there. (00:12:00)
The One With the Pediatrician - S9-E3
Visible crew/equipment: When we first see Joey and Mike at the restaurant we can briefly see a boom mic above them. (00:08:25)
The One With The Vows - S7-E21
Visible crew/equipment: When Monica and Chandler are in the coffee house after they've finished their vows, there is a microphone and another piece of equipment moving outside behind Chandler. (00:19:40)
The One With Joey's Award - S7-E18
Continuity mistake: When the student is telling Ross he is in love with him, Ross arms are crossed one way, and in another shot they are crossed another. (00:04:00)
The One After the Superbowl, part 1 - S2-E12
Continuity mistake: Brooke Shields throws water at Joey, and he gets a large spot of water on his shirt, but in the next shot the spot is much larger than before. (00:15:55 - 00:16:55)
The One With The Soap Opera Party - S9-E20
Continuity mistake: When Rachel is confronting Joey about the secret party the water bottle in her hand changes from her left to her right hand back to her left again. (00:06:10)
The One Where Joey Speaks French - S10-E13
Continuity mistake: When Joey is drinking the milk the amount in the container changes. It sometimes grows. (00:22:00)
The One with the Stoned Guy - S1-E15
Continuity mistake: When Chandler says Ross isn't speaking metaphorically, he gestures with his right hand. In the next shot, both his hands are in his pockets. (00:06:05)
The One with Two Parts: Part 1 - S1-E16
Visible crew/equipment: Just after the credits, before we see a close up of Ross, a boom mic appears at the top of the screen in the centre of the window on the right. (00:01:30)
The One with the Boobies - S1-E13
Continuity mistake: When Joey finds out that his mother knows about his dad's affair, she tells him she's fine and holds her hands to her chest. In the next shot her hands are outstretched in front of her. (00:17:10)
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.