The One with the Blackout - S1-E7
Continuity mistake: After the light comes back on and Ross is holding his heart after he sees Rachel kissing Paulo, in the next shot when Joey starts speaking to him his hand is by his side. (00:20:40)
The One Where Chandler Takes a Bath - S8-E13
Continuity mistake: When Chandler is in the bath with Monica at the end a splodge of bubbles appears on his face between shots. (00:20:40)
The One Where Ross Can't Flirt - S5-E19
Continuity mistake: When Phoebe asks Rachel for the earrings she borrowed, Chandler is visible in the background, standing up with one leg on the table and talking to Joey. When the camera angle switches, Chandler is sitting down. (00:05:00)
The One With The Triplets - S5-E3
Continuity mistake: When they enter the hospital, Phoebe says, "Someone should call Frank and Alice, and my mom," and Ross searches his pockets(presumably for his cell phone). But in the next shot, his hands are not in his pockets. (00:00:05)
The One Where the Monkey Gets Away - S1-E19
Continuity mistake: When Ross comes back to Monica and Rachel's apartment and Rachel tells him she has lost Marcel, he takes off his overcoat and lays it over the back of the sofa and then sits by it. As the scene continues, the sofa is still visible in some shots and the black coat is crumpled up on the sofa cushion. (00:11:25)
The One Where They All Turn Thirty - S7-E14
Continuity mistake: As Monica is giving her speech while drunk, at one point her hands go from her sides to gripping each other between shots. (00:00:10)
The One Where Eddie Moves In - S2-E17
Visible crew/equipment: In the scene where Monica, Joey, and Phoebe are sat on the couch in the coffee house, (whilst the camera is drifting left) a boom mic at the top of the screen is visible for a short period of time. During this time the mic can be seen moving. (00:06:50)
The One with Two Parts: Part 2 - S1-E17
Continuity mistake: When Joey is talking to Ross and Chandler about missing Phoebe's birthday, Chandler has the newspaper in his hands. In the next shot, you can see that Chandler does not move at all, then in the next shot, the paper is on the table. (00:04:35)
The One With Rachel's Inadvertent Kiss - S5-E17
Factual error: When Joey tries to find the Hot Girl, he goes up to Ross' apartment and there is a dead end. However, when he next goes there, he manages to run away in that direction. (00:03:20)
The One With The Nap Partners - S7-E6
Continuity mistake: In the Maid Of Honour sequence, Joey's head starts to move one way. The camera cuts and it's no longer moving. (00:14:00)
The One Where Chandler Can't Remember Which Sister - S3-E11
Continuity mistake: Ross is on about going to Maryangela's to Chandler. He picks his drink up, the camera cuts and he picks it up again. (00:12:45)
The One With The Cake - S10-E4
Continuity mistake: Near the end, Monica rearranges Emma's stuffed toys in order of size. When you next see them they are jumbled up, then later they return to being neatly lined up again. (00:19:40)
The One Where Ross Meets Elizabeth's Dad - S6-E21
Continuity mistake: When Monica and Chandler enter the apartment arguing about what time they were supposed to meet, Monica has her handbag strap over her right shoulder, and then takes it off. When the shot switches to Phoebe claiming to know what time they said they would meet, the handbag strap is back over Monica's right shoulder, and then she takes it off again. (00:16:40)
The One With The Inappropriate Sister - S5-E10
Continuity mistake: Ross and Joey are discussing Joey writing his own moviel; Ross sits down to begin writing, and Joey places a beer on the table next to him. The large part of the label is facing to the side, but it then switches to facing the front without being touched. (00:06:20)
Continuity mistake: During breakfast, when Joey and Phoebe are talking about driving back, the fork in Phoebe's left hand disappears and reappears. (00:03:55)
The One With The Metaphorical Tunnel - S3-E4
Continuity mistake: When Monica is telling Susan and Carol that Ross used to dress up as a girl, Ross goes over to the table and picks up a drink. When he picks it up it has a cork in it. In the next shot he has the cork already in his hand. (00:20:30)
The One With The Proposal, part 2 - S6-E25
Continuity mistake: At the end when Chandler returns to his apartment, from the outside he opens the door with his right hand. Then it cuts to an inside view and he's opening it with his left hand on the handle. (00:17:40)
The One Where Ross Moves In - S5-E7
Continuity mistake: Chandler and Joey are on their couch thinking about what to do about Ross; when Chandler says "what are we gonna do?", he switches from crossed arms to hands folded in his lap. (00:15:45)
The One With The 'Cuffs - S4-E3
Continuity mistake: Joey's encounter with the encyclopedia salesman is riddled with errors. First, when the salesman is talking about Van Gogh and saying "He painted that," he points to a page that is all text; later, Joey is reading about vomit, and it is at a pont much further on in the book than when the salesman read about vulcanized rubber (which it shouldn't be, given the encyclopedia is alphabetized); finally, when Joey buys the "V" volume, as he opens it he has a $50 bill in his hand, and when the shot changes to show the salesman he is somehow folding and pocketing it without having taken it from Joey. (00:13:00)
The One Where They're Going To PARTY! - S4-E9
Continuity mistake: When Chandler answers the phone and it's Gandalf calling to cancel, Ross is just opening the fridge, but the shot cuts to Chandler with the phone and Ross is putting two beers down on the side next to him - too quick to have got them there from the fridge. (00:05:35)
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.