The One With The Proposal, part 1 - S6-E24
Factual error: There's no charity event in the world that would be clamoring for non-paying guests and offering them free food and an open bar.
The One Where Rachel Quits - S3-E10
Continuity mistake: In the very first shot of Chandler and Rachel sitting on the couch, Chandler has his legs crossed left over right, but it cuts to the closeup and his legs are crossed right over left, then it's back to left over right.
The One The Morning After - S3-E16
Continuity mistake: When Ross is listening to Rachel's message on his answering machine, only the open pill bottle sits on the counter beside his mug, but then the bottle cap and cotton filler suddenly appear next to it. (00:03:00)
The One The Morning After - S3-E16
Continuity mistake: While Monica's preparing her shake in the blender she asks Rachel about the anniversary dinner, and when Rachel tells her that she and Ross broke up, Monica throws the blender lid and it flies over the counter, but in the next shot the lid is back in Monica's hand, and then it vanishes again. (00:01:10)
The One With The Worst Best Man Ever - S4-E22
Visible crew/equipment: After Phoebe gets very annoyed by the leather pants gift, when she stands up and walks to the bathroom the boom mic can be seen at the top of the screen, just as she pulls the crepe streamer down.
The One With The Worst Best Man Ever - S4-E22
Visible crew/equipment: After Phoebe looks down at her belly and scolds the triplets for kicking one another, when Joey walks in and asks if he can borrow a big bowl, studio lighting rigs can be seen at the top right side of the screen.
The One With Chandler In A Box - S4-E8
Visible crew/equipment: When Monica and Tim are on the balcony, we can see the end of the set wall, as well as studio equipment and lighting, at the left side of the screen.
The One With The Apothecary Table - S6-E11
Visible crew/equipment: After Rachel tells Ross that Phoebe hates Pottery Barn, when she tells him that Phoebe will be there any second and they need to cover the apothecary table, a crew member can be seen at the left side of the screen.
The One With Rachel's Inadvertent Kiss - S5-E17
Visible crew/equipment: After Monica tells Rachel that a guy from Ralph Lauren called and they want her for a second interview, when Rachel walks toward the TV and then when she stands in front of the TV, we can see a few tapes marks on the floor. One of those tape marks is also visible in one of Joey's closeups.
Continuity mistake: While Mama Joey is looking for Chick Jr. and Duck Jr., the things pinned to the bulletin board beside the door have vanished when Monica and Chandler walk in, but after Monica walks out to check on the twins everything's back up on the board like it was before.
The One With The Fertility Test - S9-E21
Visible crew/equipment: After Chandler tells Monica the results of the fertility tests, when it cuts to Phoebe as the receptionist says good morning to her, the camera can be seen at the left side of the screen.
The One Where Rachel Has a Baby, part 2 - S8-E24
Visible crew/equipment: After Rachel asks Joey to get the other box of tissues under Ross' coat, when Joey drops the ring box then picks it up and opens it we can see the end of the hospital set floor and wall, behind the chair.
The One With The Secret Closet - S8-E14
Other mistake: When Chandler stops by to see how Joey's doing after Rachel has moved out, Joey's holding a gallon jug of Minute Maid orange juice with its brand name partially painted to hide it, so it now reads "Vinut Vaio" on the label.
The One With The Prom Video - S2-E14
Continuity mistake: On the actual prom video tape, it only takes Ross a mere 12-13 seconds to run up the stairs, completely change outfits into his Dad's tux, and reappear ready-to-go on the stairway. This is impossibly fast. Also, it is not possible that the prom video tape was stopped and re-started during this time, which could account for the short duration. The reason is because Ross's mom was the one filming at this point. It turns out that she has no idea how to stop the video recorder. Thus, the video tape was one continuous recording, and we are expected to believe that Ross could change outfits within 12 seconds. (00:21:25)
The One Where Rachel Has a Baby, part 2 - S8-E24
Continuity mistake: When Ross is telling Chandler that Janice is there and she's contracting Chandler can be seen putting his hands on his face. In the very next shot his hands are by his side again. (00:02:50)
The One With Ross's Inappropriate Song - S9-E7
Other mistake: When Joey and Chandler are watching Richard's sex tape the screen is off right before Monica comes in, and yet she sees the video playing (you can still hear sex noises with the screen off). (00:14:40)
The One With Five Steaks and an Eggplant - S2-E5
Continuity mistake: When Chandler is seducing Jade in Central Perk, he turns to face Ross after handing her the tissues. The camera then cuts to another angle where he is still facing Jade, and then he turns again to face Ross. (00:06:40)
The One With Christmas in Tulsa - S9-E10
Audio problem: In the scene before the opening credits, if you listen carefully to Chandler's line, "If I don't get it done, I'll be fired", you can tell it's an overdub. The background noise and volume levels are different than the rest of his speaking. (00:00:33)
The One With the Baby on the Bus - S2-E6
Audio problem: When the lady on the street (played by Lea Thompson) is talking to Joey and Chandler, her mouth and her voice are not in sync.
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.