The One With All The Rugby - S4-E15
Continuity mistake: When Rachel is talking to Joey about flushing the toilet, her hands go from counter to bottle top to counter with no time between camera cuts. (00:02:00)
The One Where Chandler Takes a Bath - S8-E13
Continuity mistake: When Chandler takes hold of the boat he grabs the middle of it, the camera changes angle and he is now holding it at the end of the boat. (00:06:20)
The One Where Ross Dates a Student - S6-E18
Continuity mistake: When Rachel and Joey are at Joey's apartment throwing wet paper towels at the entertainment unit, Phoebe comes in to ask Rachel to switch back to Monica's apartment. During the scene, Phoebe is standing in front of the entertainment unit, and the wet paper towels' number and positions change between shots. (00:18:10)
The One With The Dollhouse - S3-E20
Continuity mistake: When Joey is talking to the understudy, he is holding a cup. It disappears right when she invites him for a drink. (00:07:20)
The One With Ross's Wedding, part 2 - S4-E24
Continuity mistake: In the episode where Rachel goes to tell Ross she loves him (in England) she leaves her room with a brown sleeveless vest and a cardigan over her shoulders. The cardigan then disappears in all other shots until after Phoebe says "he's in love with the British chippy". Her leather jacket also goes from being in her hands to over her arm. (00:19:30)
The One With Ross's Grant - S10-E6
Visible crew/equipment: When Joey turns the video off, the end of the set is visible in the TV. (00:15:10)
The One Where Joey Tells Rachel - S8-E16
Continuity mistake: When Monica is talking to Ross about how Joey feels, there is a shot of Monica bending down to talk to Ross, and some magazines on a table. At the end of the scene the magazines have changed place. (00:05:30)
The One Where Monica Gets a Roommate - S1-E1
Continuity mistake: When Phoebe cleanses Ross' aura, he holds up both hands to stop her. In the next shot his left hand is lowered. (00:02:50)
The One Where Monica Gets a Roommate - S1-E1
Continuity mistake: When Monica asks Ross, 'Do you want me to stay?', her hands are in front of her, not touching Ross. In the next shot, her right hand is holding onto Ross's wrist. (00:09:10)
The One with the Sonogram at the End - S1-E2
Continuity mistake: When we first see Ross at the museum, the woman he works with is holding a rock. In the first shot, she is holding onto it with her right hand at the bottom and just before the shot changes she turns away from Ross. In the next shot, she is facing Ross again and her right hand has moved on top of the rock. (00:02:05)
The One With The Fertility Test - S9-E21
Continuity mistake: When Charlie tells Ross about the Met, Ross goes sitting next to her, and his hands are empty when he says: 'Joey really knows his art, huh?' When Charlie says: 'Not so much' Ross suddenly holds a book in his hands. Then, when the shot changes, the book is gone again. It keeps (dis)appearing during the scene. (00:20:35)
The One With The Metaphorical Tunnel - S3-E4
Continuity mistake: When Ross shows G.I. Joe to Ben, the Barbie's legs are pointing towards the refrigerator. In the next shot, they are suddenly pointing the opposite way. (00:13:50)
The One Where Everybody Finds Out - S5-E14
Continuity mistake: Monica and Chandler are deciding what to do about Phoebe's flirting; the shot cuts to her and Rachel, and Rachel's right hand is on her chin. In the next shot, it's her left hand on her chin. (00:12:00)
The One Where Ross Hugs Rachel - S6-E2
Continuity mistake: In the scene where Rachel is all happy that the three of them are going to live together, her arms keep changing position throughout the scene. (00:08:35)
The One with Two Parts: Part 2 - S1-E17
Continuity mistake: After Ross drops the cake, when everyone gathers around the table, there is a shot of Monica with no one behind her. In the next shot, Phoebe has appeared next to the lamp behind Monica from nowhere. (00:06:30)
The One with the Blackout - S1-E7
Continuity mistake: At the end of the episode when Chandler watches Jill leave the bank, his face is pressed up against the centre of the glass door. In the next shot his face is leaning on the metal edge around the glass. (00:21:20)
The One With Chandler's Dad - S7-E22
Continuity mistake: When Joey is talking about all the different fabrics, the magazine Phoebe is holding keeps changing position. (00:17:00)
The One With Christmas in Tulsa - S9-E10
Continuity mistake: When the group notice that it's snowing outside, external shots show a lot more snow than the internal shots. (00:19:20)
The One with the Boobies - S1-E13
Continuity mistake: When Phoebe is talking to Roger in Central Perk when she tells him her friends don't like him, her hair tie is half way down her back. When Phoebe tells him she doesn't think he's creepy, her hair tie is just above her shoulder. (00:19:05)
The One With The Dirty Girl - S4-E6
Visible crew/equipment: When Monica leaves the kitchen into the next scene, the boom mic is visible at the top of the screen above the kitchen door for a short period of time. (00:07:40)
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.