Continuity mistake: In the pilot episode, the guy Rachel left at the altar is called Barry Finkle. In all the other episodes his name is Barry Farber.
The One With Joey's Porsche - S6-E5
Revealing mistake: The triplets are supposed to be two girls and a boy; however, certain revealing shots of their anatomy when they are in Monica's apartment show that the two babies playing the girls are actually boys.
The One Where Joey Dates Rachel - S8-E12
Visible crew/equipment: When Ross is wearing roller-blades while teaching his class, just as he points to the map and says, "Here," the second time, there's a hidden crew member that can be seen behind the end of the set wall with the map, at the left side of the screen.
The One With Phoebe's Wedding - S10-E12
Continuity mistake: When Monica and Phoebe are talking about having a harpist versus Marjorie playing the steel drums, the handbag strap over Monica's shoulder vanishes and reappears.
The One With Phoebe's Birthday Dinner - S9-E5
Continuity mistake: Phoebe implies that her birthday is round about Halloween - October 31st. However, in The One With Frank Jr in series three, she tells Frank that her birthday is February 16th. (00:00:05)
The One with All the Poker - S1-E18
Visible crew/equipment: The friends are all at Ross' place. Marcel (the monkey) is dancing to the music just before they all play poker (before Rachel enters). However, the reflection of the animal trainer directing Marcel is visible in the TV screen. (00:11:45)
The One After Joey And Rachel Kiss - S10-E1
Visible crew/equipment: After Chandler starts removing Monica's braids from the shower curtain rings, when it cuts to Joey and Rachel walking into their apartment, we can see the hole in the wall at the right side of the screen. And it's seen again when Ross walks in on Rachel and Joey kissing.
The One With Joey's New Brain - S7-E15
Character mistake: After Susan Sarandon slaps Joey the first time, after he tells her that her character is dying on DOOL, she turns away and right before she is off camera she smiles, instead of maintaining her anger. (00:06:05)
The One Where Emma Cries - S9-E2
Continuity mistake: In the coffee house, when Ross and Joey are having a fight about Joey's proposal to Rachel, Ross does the "quote gesture" with his hands and Joey says he doesn't know what this means. In TOW the Secret Closet, Joey used it himself when talking to Chandler, in the scene where he asks him to bring a bobby pin to pick the closet's lock. (00:08:55)
Continuity mistake: Mr Heckles's apartment number changes from 8 to 5.
The One with the Dozen Lasagnas - S1-E12
Visible crew/equipment: When Rachel's dumping all of Paolo's clothes over the side of the balcony, we can see the apartment's back wall is only half a wall, with the sound stage and equipment at the left side of the screen.
The One Where Ross Is Fine - S10-E2
Factual error: If Ross really was holding the sizzling fajitas like that for as long as he was without any gloves, he would be in the emergency room.
The One With The Prom Video - S2-E14
Visible crew/equipment: After Chandler throws the basketball and breaks the lamp, in the next shot of Joey as he mentions the bracelet, we can see the reflection of a crewmember (standing behind the foosball table) on the left side of the TV screen (Chandler's reflection is at the center of the screen).
The One Where Ross and Rachel...You Know - S2-E15
Visible crew/equipment: While "The Dick Van Dyke Show" is playing on TV, when Rachel walks in and heads over to Ross, a crewmember can be seen at the left side of the screen, behind Phoebe.
The One With All The Haste - S4-E19
Visible crew/equipment: After Joey convinces Chandler to agree to the bet, when Phoebe walks over to the recliners and tells everyone that she has a game, the camera can be seen moving at the right side of the screen. (DVD).
The One With The Fake Party - S4-E16
Visible crew/equipment: While Rachel is talking to Chandler she tries to get Joshua's attention so she pretends to laugh, and in the shot facing Joshua the end of the set wall can be seen, with the brick set wall from Central Perk on the other side.
The One With Joey's Dirty Day - S4-E14
Visible crew/equipment: After the exterior shot of Bloomingdale's, when it cuts to Rachel helping Joshua with his clothes the camera can be seen at the left side of the screen.
The One With Phoebe's Cookies - S7-E3
Visible crew/equipment: When Chandler and Jack, both wearing robes, are walking toward the whirlpool room, just as Chandler tells Jack that he'd feel more comfortable if Jack didn't tell anyone what had happened, we can see the shadow of the boom mic moving across the wall at the right side of the screen.
The One with the Monkey - S1-E10
Visible crew/equipment: While Phoebe's talking about David she mentions the movie 'An Officer and a Gentleman, ' and when she tells them that David is kind of like the guy she went with to see that movie, in the following wideshot the boom mic can be seen moving around at the top of the screen. (00:06:10)
The One with the Monkey - S1-E10
Visible crew/equipment: When Joey walks into Monica's apartment dressed as an elf, just as he takes off his coat we can see stage lighting at the top left side of the screen, above the large dangling star.
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.