The One with the Dozen Lasagnas - S1-E12
Visible crew/equipment: When Chandler and Joey are talking about buying a table, a line of stage lights is visible in the tv. (00:07:25)
The One With Rachel's Date - S8-E5
Visible crew/equipment: As Phoebe walks into the restaurant kitchen, a crewmember's hand is visible stopping the door from swinging too much. (00:16:50)
The One Where Monica and Richard Are Just Friends - S3-E13
Continuity mistake: Monica and Richard are squishing tomatoes; when she starts dabbing his shirt with club soda the door behind them leading to Monica's room goes from open to shut between two shots. (00:08:30)
The One Where Estelle Dies - S10-E15
Continuity mistake: When Chandler says his boss is getting him a new lamp in his cubicle, his arm is over the back of the chair, in the immediate next shot it is moved and then back again. (00:00:50)
The One with the East German Laundry Detergent - S1-E5
Continuity mistake: When Joey and Angela talk in Central Perk, the black waitress with long hair is visible the background of shots of Angela. When we see shots of Joey the same waitress is also behind him near Gunther. She is in two places at once. (00:05:50)
The One with the Thumb - S1-E3
Continuity mistake: When Chandler says he won't quit smoking, he holds his hands out. In the next shot, they are lowered between his legs. (00:06:00)
The One Where Rachel Quits - S3-E10
Continuity mistake: The gang enters Monica's apartment and it is full of dead Christmas trees. After everybody has entered the apartment we see that the apartment door is shut; the shot then changes to the gang around the sofa, but in the next shot when Rachel answers the phone we can see behind her that the door is open. (00:19:00)
The One with the Dozen Lasagnas - S1-E12
Continuity mistake: In The One Where Ross Finds Out he's having a boy (series 1), just before Rachel lets it slip, she is sitting on the edge of a chair eating a cookie. Watch her dress, it changes back and forth from being undone from the waist up, to only having two buttons open at the top. (00:19:55)
The One with the Stoned Guy - S1-E15
Continuity mistake: Just before Chandler tells everyone he quit, he holds out both hands in front of him. In the next shot, his left hand is by his side. (00:02:05)
The One with the Stoned Guy - S1-E15
Continuity mistake: When Chandler says he can't work out what he wants to do working at his job, he is standing behind the sofa. In the next shot, he is in front of the sofa sitting down. (00:02:50)
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 the Stoned Guy - S1-E15
Continuity mistake: In the scene where Monica is explaining to Rachel that she has hired her friend Wendy as her waitress, Rachel is holding a water bottle in one hand and the cap in the other. Monica moves towards her and Rachel is pointing her hand in Monica's direction and then in the next shot we see Rachel from another angle and her hand is holding the cap on top of the bottle too fast to have happened in normal time. (00:08:40)
The One Where Chandler Doesn't Like Dogs - S7-E8
Revealing mistake: When Rachel and Tag are in the hall at the end of the episode, the tan mark from Jennifer Aniston's wedding ring is visible on her hand. (00:19:15)
The One With Monica and Chandler's Wedding, Part 2 - S7-E24
Continuity mistake: The way Phoebe holds her flowers when told to smile changes depending on the angle. (00:04:30)
The One with Mrs. Bing - S1-E11
Continuity mistake: When they are all eating at the restaurant and Mrs. Bing is telling Rachel how to write a book, the amount of food on Rachel's plate changes between shots without her touching it. (00:06:55)
The One Where Monica Gets a Roommate - S1-E1
Continuity mistake: At the end of the episode, as a customer in Central Perk asks Rachel for more coffee, Chandler has set his cup of coffee on the table. In the next shot, he is holding it again. (00:28:05)
The One with Two Parts: Part 2 - S1-E17
Continuity mistake: Just before Ross drops the cake, Rachel is pouring crisps into a bowl with her left hand holding the bottom of the bag. In the next shot, she is holding the bottom of the bag with her right hand. (00:05:50)
The One with the East German Laundry Detergent - S1-E5
Continuity mistake: When Rachel is holding the ice pack to Ross's head at the end of the episode, Ross' left hand is lowered, then in the next shot, it is raised to his head. (00:21:35)
The One with the East German Laundry Detergent - S1-E5
Continuity mistake: When Rachel says 'everything looks like jammies now' she is holding up a t shirt. In the next shot as she throws it down, it is now folded up. (00:17:00)
The One with the Monkey - S1-E10
Continuity mistake: Just after Chandler explains the situation to Janice, she goes off crying, and Ross takes a photo. The flash is much brighter than it should be - the screen's whited out to disguise a jump cut - after the flash Chandler's noticeably further back than he was before. (00:17:45)
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.