The One Where Chandler Gets Caught - S10-E10
Continuity mistake: After Chandler says he loves the house, Monica says "this is huge" with her arms by her sides. We then cut to a front shot, and Monica's holding her hands in front of her. (00:04:30)
The One with the Sonogram at the End - S1-E2
Continuity mistake: At the very start of the episode, Monica holds up both of her hands. In the next shot her right hand is wrapped around her legs. (00:00:05)
The One With The Cake - S10-E4
Continuity mistake: When Ross and Rachel are driving back, Ross starts to put the seatbelt on. Instant cut, and he has his seatbelt on. (00:17:35)
Continuity mistake: In addition to Ross remaining 29 for several years, he also gets younger as the years go by. When he marries Emily he is thirty (Monica describes herself as "a single mother with a 30 year old son"), but when he divorces her he is twenty-nine.
Revealing mistake: When the first baby is born, Monica and Chandler are asked by the doctor if they would like to cut the umbilical cord. However, just before this line is delivered, the baby is moved and its navel is visible.
The One with the Candy Hearts - S1-E14
Continuity mistake: Just after Rachel throws a bread roll at Joey, he turns to Chandler to speak to him. Chandler is holding his hands out in front of him. In the next shot, his right hand is touching his face, then in the next shot, it is lowered again. (00:02:55)
The One With The Blind Dates - S9-E14
Visible crew/equipment: When Phoebe meets Joey to tell him about the date for Rach, the stage lights are seen outside the coffee-shop. (00:05:35)
The One Where Nana Dies Twice - S1-E8
Continuity mistake: In this episode, Rachel tells Chandler that she thought he was gay until he spent Phoebe's entire birthday party talking to her breasts. But at this point since Rachel joined the group, Phoebe hasn't had a birthday yet; that occurs some episodes later, in "The One With Two Parts." (00:02:00)
The One Where Rachel Tells... - S8-E3
Continuity mistake: Joey hits the blue robots head down. The camera changes and the head is back up. (00:07:25)
The One With Five Steaks and an Eggplant - S2-E5
Plot hole: Chandler has always loaned Joey money for various things (rent, acting classes, Christmas gifts, etc), so he shouldn't be surprised to learn that Joey doesn't have money for fancy dinners or birthday celebrations. Plus, Joey never seems to have a problem taking money/handouts from Chandler - why now? Also, Chandler explains that they're buying Ross a cake ($20), but why doesn't Monica (the chef and his sister) just make him one? (00:07:50)
The One Where Nana Dies Twice - S1-E8
Other mistake: When Ross is helping choose a dress and shoes for Nana to wear, he is behind the chest of drawers in the cupboard. He looks in a shoebox and finds lots of pink sweet'n low packets, then pulls down another box and lots of packets fall down on him. However, it is obvious that someone is above him showering the packets down, as they are coming from directly above him, not from on top of the door. (00:12:35)
The One With Ross' New Girlfriend - S2-E1
Other mistake: After Rachel takes the phone from Ross and hangs up on Julie, when she's sitting at the table there's a bottle of Evian water in front of Monica with part of its brand name covered - the letters "e" and "n" are whited out leaving "via" on the bottle.
The One With Rachel's Other Sister - S9-E8
Character mistake: While watching the parade near the beginning, Chandler gets excited because there's a band from Muskogee, OK, which is supposedly only four hours from Tulsa, where he works. It's actually less than an hour away from Tulsa.
The One With All The Jealousy - S3-E12
Continuity mistake: In the coffee house when Rachel is talking to Ross about trust, a couple comes in behind her and stands at the bar. In the next shot of Rachel they have switched positions; then they are gone, and you never see them walk past Chandler. (00:19:05)
The One With The Blind Dates - S9-E14
Continuity mistake: When Rachel is at her blind date, and Steve tells her that he's selling T-Shirts now, Rachel's arms are on the table, but then the camera cuts to her, and she has her arms crossed. (00:12:00)
The One with Two Parts: Part 1 - S1-E16
Continuity mistake: When Ross says he's going to be a father, Chandler drinks from his cup. In the next shot the cup is gone. (00:16:55)
The One With The Joke - S6-E12
Continuity mistake: At the end Rachel turns away from Gunther and pulls her sleeves above her elbows. Next shot they are below her elbows. (00:20:00)
The One With The Red Sweater - S8-E2
Continuity mistake: When Ross is explaining about buying more cameras, the way he holds it changes. Follow the lens. (00:08:40)
The One With Mac & C.H.E.E.S.E. - S6-E20
Continuity mistake: When Chandler answers Joey's phone there is a drawing on the magnadoodle. However in the next shot it has vanished with no time. (00:17:20)
The One With The Bullies - S2-E21
Continuity mistake: When the bullies are about to fight with Chandler and Ross, they decide to put all watches and keys into a hat. He takes the hat off and places it down and everyone puts the watches and keys in. The shot changes to a further out view and the bully has a different hat on his head while another hat is still placed where he put it. When there is a closer view of him, there is once again no hat on his head. (00:19:30)
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.