The One With Ross's Sandwich - S5-E9
Continuity mistake: In the scene where Joey has just brought home his date, he jiggles the door handle, which makes Monica and Chandler run off into Chandler's room. Chandler kicks a circle cushion off the mat, but when the camera cuts back, it's on the mat again, and then off the mat again when Rachel walks in. A red cushion also moves. (00:09:10)
The One With Rachel's Going Away Party - S10-E16
Continuity mistake: The level of beer in Ross' bottle goes up and down between shots as he's talking to Joey at the kitchen table. (00:12:00)
The One With Ross's Denial - S6-E3
Continuity mistake: When Monica and Chandler are eating, Monica's plate holds a sandwich and chips. When Joey comes in, Monica's plate contains no chips, and when he leaves the chips are back again. (00:04:00)
The One With The Cake - S10-E4
Visible crew/equipment: When Monica is talking to Chandler about waking Emma up, as she walks over to him, crew lights are visible in the top left corner of the screen. (00:05:35)
The One With Phoebe's Cookies - S7-E3
Continuity mistake: While trying out different recipes, at one point Monica's holding a cookie in her hand. It's a whole cookie, but when the angle changes, suddenly a chunk is missing. (00:13:00)
The One With All The Thanksgivings - S5-E8
Continuity mistake: In the scene where they take Chandler to the hospital, Monica hands the bag of ice containing Chandler's toe to the doctor. The doctor is wearing gloves in one shot then no gloves in the next. This goes back and forth three or four times. (00:07:45)
The One With Joey's Award - S7-E18
Continuity mistake: When Joey sets down the award he sets it at a straight angle to the counter edge, but when Joey catches Rachel about to pick it up it is angled and not straight. (00:13:45)
The One With The Girl Who Hits Joey - S5-E15
Other mistake: When Ross and Phoebe are eating cheese puffs and party mix out of the bags, Phoebe's bag reads "Heez-I" instead of Cheez-It, and the brand "Sunshine" has been covered.
The One Where Estelle Dies - S10-E15
Revealing mistake: When Chandler and Monica are at the door of the house you can see that it is a fake background behind Chandler. (00:09:35)
The One Where Estelle Dies - S10-E15
Continuity mistake: The second time Ross visits Mr. Zelner, he hands him a pterodactyl egg replica. The hole in the egg switches between the top or bottom, depending on the shot. (00:14:40)
The One With The Flashback - S3-E6
Continuity mistake: Phoebe and Ross are in the soon to be closed bar sitting on the pool table. He clears it of everything, but a few seconds later he stops making out because the "damn balls are in the way" and somehow magics up a handful of snooker balls. If you look closely when he's clearing the table, there are two dark balls at the side which he studiously ignores, and which Lisa Kudrow then surreptitiously grabs and takes onto the table with her so David Schwimmer can grab them later. (00:19:35)
The One With All The Cheesecakes - S7-E11
Continuity mistake: Chandler walks into his apartment to find Rachel eating cheesecake without him. As the door is shutting, the pen on the Magnadoodle is hanging loose; it can be heard hitting the door when Chandler is closing it. The next shot shows Rachel stop eating, and when we next see Chandler, the door is closed and the pen is in its holder. (00:15:50)
The One With All The Wedding Dresses - S4-E20
Continuity mistake: When Chandler tells Joey that he should go to a sleep clinic, Monica puts her coffee cup on the table, but then it reappears in her hands and disappears repeatedly. (00:02:00)
The One With The Embryos - S4-E12
Visible crew/equipment: When Rachel crosses the boys' apartment to see what is in the drawer, if you look up to the right, you can see a glimpse of a boom mic. (00:20:25)
The One with the Thumb - S1-E3
Continuity mistake: When Phoebe visits Lizzie, Lizzie is feeding the pigeons with her right hand. In the next shot, a plastic bag has appeared in her left hand from nowhere. (00:11:40)
The One With Chandler In A Box - S4-E8
Continuity mistake: In the episode with Chandler in a box, when Rachel is showing Ross all the things she had kept from their relationship in a shoe box, her hair is tucked behind her ear in most shots, but when she says "I keep the gifts that matter", it's all loose, then goes back to how it was in the following shot. Also in the shot before that line she's pointing the horn fossil's tip at Ross, but when it cuts to a front view she's holding the horn's tip and pointing the broken end. (00:16:30)
The One Where Emma Cries - S9-E2
Continuity mistake: When Ross and Rachel are talking about which muffin she wants, her sleeves move up and down her arms with the different shots. (00:04:45)
The One With All The Candy - S7-E9
Continuity mistake: When Phoebe sits on the swing (when learning to ride her bike), her scarf has one end down her front and the other over her left shoulder. In the next shot, the end is over her right shoulder instead of her left. A couple of shots later, it is switched back again. (00:11:05)
The One Where Chandler Can't Cry - S6-E14
Continuity mistake: When Chandler throws his copy of "Chicken Soup for the Soul" under the sofa and it goes across the floor to Monica's feet, she kneels down to pick it up by the counter next to the door, but when she rises, she's next to a chair that's under the table. (00:12:20)
The One with the Stoned Guy - S1-E15
Continuity mistake: When the stoned guy eats all the shrimp ravioli, he eats everything including the garnish. When we next see the plate, the garnish has returned. (00:17:30)
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.