The One With The Free Porn - S4-E17
Continuity mistake: In the beginning when Rachel says "Do we really have to watch this while we eat." From the front view Rachel's arms are crossed and then from the back their are by her side and back with no time to turn.
The One With The Tiny T-Shirt - S3-E19
Continuity mistake: When Ross is looking through the peephole, his right foot moves inward with no time to move.
Visible crew/equipment: Most of the time when they show the Central Perk logo on the window. If you look inside you see the set because the wall doesn't go far enough.
The One Where Ross Is Fine - S10-E2
Continuity mistake: When Owen is telling his parents Chandler paid him $50 not to tell them he knows he is adopted, Chandler says "which technically now you should give back" and he is looking at Owen. In the next shot his back is to Owen.
The One Where Rachel's Sister Babysits - S10-E5
Continuity mistake: When Ross is looking at Emma in the coffee house he turns and looks at the guys on the couch and mouths the word "what." Then in the next shot he does it again.
The One With Joey's Interview - S8-E19
Continuity mistake: In the opening scene, Joey is holding the Soap Opera Digest magazine. In one shot it is open but folded, then in another it is open all the way, then it's folded again, then it's open again, then it's closed, and then it's folded again. The change is never shown.
The One With All The Haste - S4-E19
Continuity mistake: The singing neighbor is more than halfway done tying his tie but when Rachel yells at him, the tie is merely draped around his neck.
The One With All The Cheesecakes - S7-E11
Continuity mistake: When Ross and Monica are at the wedding reception talking to the other guests at their table, a lady wearing a red scarf is standing behind them with her back turned. Throughout the scene the red scarf keeps changing position very quickly - in one shot it's slid down one shoulder, then back up in the next and then slid down again. At the end of the scene it's off her back entirely and she has just slung it over her arms. (00:12:15 - 00:13:15)
The One With Rachel's Inadvertent Kiss - S5-E17
Continuity mistake: When Rachel returns from her first interview at Ralph Lauren she takes out a bottle of water out of the fridge. In the following shot her hands are empty and there are only Chandler's and Joey's bottles on the table. Rachel's bottle reappears on the table next to Chandler's and Joey's bottles (that are also standing closer together than before) for a moment just when Chandler starts saying "Well, you could have tried not kissing him" and then vanishes again. (00:06:20 - 00:07:15)
Continuity mistake: Joey's bedroom door opens and closes as the camera goes between him and Monica and Chandler when he is telling him he doesn't want to see them kissing.
The One With Rachel's Inadvertent Kiss - S5-E17
Continuity mistake: Joey is watching Ross from Monica's window and goes searching for the hot girl. The exterior shot crossing the street between buildings is inconsistent with the interior shots, which show them looking at each other across an alley with no visible fire escapes. The exterior shot crosses a street to a building with street-facing fire escapes on every apartment.
The One With The Lottery - S9-E18
Continuity mistake: Joey's thumb on the wishbone changes positions as the camera switches between Joey and Monica.
The One with the Candy Hearts - S1-E14
Continuity mistake: In the scene where Ross and Carol are talking and Carol says that all he needs is to find a woman who likes men, a woman comes across the room walking in high heels. The shot is wrong because we see her up until she's half the way through the table, and the next shot shows her four steps back. (00:23:10)
The One With The Football - S3-E9
Continuity mistake: When they are talking about the Geller Cup, Monica is looking at Ross and then looks down. But when the camera angle changes, she is still looking at Ross and then looks down again.
The One With The Giant Poking Device - S3-E8
Continuity mistake: When Chandler is breaking up with Janice and she walks away trying to shake him off, her jumper is untied. When Chandler says "You can't go, I have your shoe" we cut back to Janice and her jumper has been tied at the bottom.
The One With The Giant Poking Device - S3-E8
Continuity mistake: When Rachel and Monica are stressing about the bump on Ben's head, Rachel's hair keeps moving from tucked behind her ear to hanging loosely in her face between shots.
The One With The Metaphorical Tunnel - S3-E4
Continuity mistake: In the scene where Chandler is sitting on the cabinet talking to Rachel and Monica, Rachel is holding an ice-cream bucket and a spoon. However, they keep showing up on the counter and then back in her hands between shots.
The One With The Dirty Girl - S4-E6
Continuity mistake: Rachel is working on the crossword puzzle and her legs are crossed (first left over right) and they switch back and forth between shots (about four times) without any time to do so.
The One With The Inappropriate Sister - S5-E10
Continuity mistake: When Joey and Phoebe are talking to each other on the couch, Rachel tucks her hair behind her ears, but in the next shot it's moved.
The One With The Inappropriate Sister - S5-E10
Continuity mistake: When Joey is talking to Ross about him being an actor, the shot changes and Ross is holding his beer in a different position.
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.