The One with the Sonogram at the End - S1-E2
Continuity mistake: When Monica holds the glass pan of lasagna up over her head to try and find Rachel's ring, when it cuts to the next shot the lasagna is still in the air, but it's different - note burned corners, etc. (00:06:10)
The One With The Metaphorical Tunnel - S3-E4
Continuity mistake: When Ross is singing to Ben about GI Joe, there are two plates in the dish drainer behind him, but when Joey walks in and sits down the white plate has vanished.
The One Where Joey Moves Out - S2-E16
Continuity mistake: When Joey and Chandler are at Joey's colleague's brunch, you can see a big wall clock in some shots. You can see it when Chandler says: "Wow, there's my fantasy's come true" with the minute hand at five to the hour. Just a few seconds later, when Joey says: "Can you see me in a place like this?" it can be seen when Chandler is in shot, and the minute hand is now at 20 to the hour. (00:05:40)
The One With Joey's Interview - S8-E19
Continuity mistake: In the opening scene Rachel is holding a white felt pen in her hands and gesturing with it while she talks about the soap opera digest magazine. Between shots the pen sometimes has its cap on and then it's off again so you can see the blue tip. (00:00:20)
The One with George Stephanopoulos - S1-E4
Continuity mistake: When Ross wrestles his puck away from the little kid in the emergency room, it goes flying at the receptionist. The shot cuts, and it hits her from a completely different angle.
The One With Ross's Wedding, part 2 - S4-E24
Continuity mistake: Joey is in their hotel room asking Chandler where Monica is, and the open bathroom door behind Chandler alternates from wide open to halfway open between shots.
The One With Rachel's Going Away Party - S10-E16
Continuity mistake: When Ross fears that Rachel is going to talk with him, the volume and the color of his beer changes between shots.
The One With Ross's Library Book - S7-E7
Continuity mistake: When Monica says, "She has beaten me, Chandler, I'm tired," she puts her hands on him and they are in fists, then they are flat out, then in fists again.
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 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 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 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)
The One with the Sonogram at the End - S1-E2
Continuity mistake: When Ross arrives at Monica's apartment, both Ross and Monica walk past the beam in the middle of the room. In the next shot as they continue walking they walk past the beam again. (00:07:05)
The One With The Princess Leia Fantasy - S3-E1
Continuity mistake: In the scene where Janice tells Joey they should spend some quality time together, the position of her hair changes from in front of her shoulders to behind several times between shots. (00:13:20)
The One With The Free Porn - S4-E17
Continuity mistake: When Joey is asking Ross why he told Emily that he loves her, Joey's bottle of juice is right in front of him. In the next shot of Joey, there's two bottles - Monica's Diet Snapple and Joey's juice, and in the next shot it's back to one again. Monica's drink appears and disappears throughout the scene. (00:11:25)
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.