The One Where Monica Gets a Roommate - S1-E1
Continuity mistake: While Joey and Chandler are at the apartment with Ross, a watch suddenly appears on Joey's wrist when he announces that he and Chandler are done with the bookcase, then the watch vanishes and reappears again in the following shots at the apartment. (00:15:40)
The One with George Stephanopoulos - S1-E4
Visible crew/equipment: When Monica's looking out the window at George Stephanopoulos with the binoculars, the end of the set wall and overhead studio lighting can be seen at the left side of the screen. (00:15:20)
The One With Ross's Wedding, part 1 - S4-E23
Visible crew/equipment: After Ross and Monica talk about Emily's dream of the perfect wedding and perfect guy (and Monica's pillowcase), when it cuts to Phoebe and Rachel at the apartment we can see a crew member (wearing a blue shirt) at the left side of the screen, before Phoebe mentions that she thought of a good flaw. (DVD).
The One With Ross's Wedding, part 1 - S4-E23
Visible crew/equipment: When Joey, wearing the Union Jack hat, walks into the hotel room while Chandler's sitting on the bed, on the widescreen version we can see the end of the set wall at the right side of the screen. On the DVD, just as Joey walks across the room studio lighting can be seen at the top left side of the screen.
The One With The 'Cuffs - S4-E3
Visible crew/equipment: At the end, when Rachel walks into Joanna's office to find her handcuffed to the chair, the camera can be seen in front of the filing cabinet, at the right side of the screen.
The One With The Free Porn - S4-E17
Visible crew/equipment: After Emily says thank you to Ross and leaves, when it cuts to Chandler and Joey's apartment Rachel puts a tea bag in her mug, and in the wideshot as she says, "He told me," the camera can be seen at the left side of the screen.
The One With The Free Porn - S4-E17
Visible crew/equipment: After Joey asks Ross to buy him a Toblerone just as Ross heads out to the airport, when it cuts to the shot of Rachel and Phoebe walking into the apartment, the camera can be seen at the right side of the screen.
The One With Phoebe's Cookies - S7-E3
Other mistake: When Monica realizes that the cookies were the Nestle Toll House recipe, Monica tosses the yellow bag of Nestle brand chocolate chips to Phoebe, and the brand name has been partially covered so it looks like "I Iest" and the Toll House logo on the side has also been partially covered.
The One With Rachel's Book - S7-E2
Continuity mistake: When helmet-wearing Vicar Joey taunts Rachel one too many times she turns the tables on him, so when Joey backs away from Rachel he gets pushed up against the front door, and although he can't move, in the shot facing Joey he's at the left side of the door, but in the side shots he's at the right side of the door.
The One With Mac & C.H.E.E.S.E. - S6-E20
Continuity mistake: After Chandler has written the message on the Magna Doodle, the sliding eraser moves from the right to left side just as he leaves with Monica, and when Joey and Chandler are playing foosball, the barely legible "Joey" is now neatly written on the Magna Doodle, and its sliding eraser is again back to the right side.
Visible crew/equipment: When Chandler points out to Joey that his oblivious comment was made in front of Ross it cuts to a closeup of Ross, and camera equipment can be seen at the right side of the screen, and it's still there in the next two closeups of Ross.
The One Where Chandler Can't Cry - S6-E14
Continuity mistake: When they're watching the vampire video, Monica, Chandler, and Rachel are sitting on the couch, but in the closeups of the TV we can see the couch in the reflection and it's empty.
The One In Vegas, Part 1 - S5-E23
Continuity mistake: After Joey tells Chandler about his identical hand twin and plans for getting rich, when he sees Chandler's reaction Joey asks, "Are you unsupporting me again?" while he lifts his left arm and points accusingly at Chandler, but next shot it's Joey's right arm that's up and pointing.
The One Where Ross Can't Flirt - S5-E19
Other mistake: While everyone's watching Law and Order for Joey's big scene, Rachel, Phoebe, Monica, and Chandler are sitting on the couch and Ross is standing behind them, but in the TV's closeup the screen reflection reveals an empty couch.
The One With Rachel's Inadvertent Kiss - S5-E17
Visible crew/equipment: While Joey's looking for the "hot girl" the second time, after he knocks on the little girl's door and asks if her mother, sister, or babysitter are a hot girl, he runs off when she shouts for her father, and when Joey stops in another hallway full of doors the shadow of the moving boom mic can be seen on the door beside him.
The One With Rachel's Inadvertent Kiss - S5-E17
Visible crew/equipment: When Rachel gets back from her first interview, just as Chandler says, "Oh hey," we can see the boom mic at the top of the screen.
The One With Rachel's Inadvertent Kiss - S5-E17
Visible crew/equipment: When Joey is standing at Monica's window waving to Ross, the end of the set wall with movement on the soundstage can be seen at the left side of the screen.
The One Where Everybody Finds Out - S5-E14
Visible crew/equipment: When Ross is using the binoculars to watch Ugly Naked Guy's apartment, the end of the set wall can be seen behind him.
The One Where Everybody Finds Out - S5-E14
Visible crew/equipment: When everyone's looking out Monica's window into Ugly Naked Guy's apartment, and see all the gift baskets he's been sent, the end of the set wall in Monica's apartment can be seen.
The One Where Everybody Finds Out - S5-E14
Visible crew/equipment: While Phoebe and Rachel are at Ugly Naked Guy's apartment, when Phoebe looks out the window and sees Monica and Chandler the end of the set wall in Monica's apartment can be seen.
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.