The One with the Sonogram at the End - S1-E2
Continuity mistake: When Ross arrives at Monica's apartment, both Ross and Monica walk past the beam in the middle of the room. In the next shot as they continue walking they walk past the beam again. (00:07:05)
The One With The Princess Leia Fantasy - S3-E1
Continuity mistake: In the scene where Janice tells Joey they should spend some quality time together, the position of her hair changes from in front of her shoulders to behind several times between shots. (00:13:20)
The One With The Free Porn - S4-E17
Continuity mistake: When Joey is asking Ross why he told Emily that he loves her, Joey's bottle of juice is right in front of him. In the next shot of Joey, there's two bottles - Monica's Diet Snapple and Joey's juice, and in the next shot it's back to one again. Monica's drink appears and disappears throughout the scene. (00:11:25)
The One With The Inappropriate Sister - S5-E10
Continuity mistake: Joey and Chandler are playing 'Fireball'; Joey puts his hand through the wall next to the fridge, then in the next scene the hole is gone with no trace. (00:08:50)
The One Where Paul's The Man - S6-E22
Other mistake: There are two mistakes in the scene where Phoebe spots Joey's picture in the laundromat. For one thing, when Phoebe says "Oh, here it is", she is looking in the opposite direction of the picture. Also, when she says this, her mouth doesn't move at all. (00:16:10)
The One With Rachel's Date - S8-E5
Visible crew/equipment: When Phoebe is telling Monica about Tim, there is someone in the background with a black mug that is reflecting some of the stage lights. (00:05:20)
The One With The Video Tape - S8-E4
Continuity mistake: When Rachel is finding out that people know the Europe story, hers legs keep changing positions. (00:19:15)
The One with the Dozen Lasagnas - S1-E12
Continuity mistake: When Ross visits Carol, he hugs her, and puts his hands on her hips. In the next shot, he isn't touching her and has taken a step back instantly. (00:05:10)
The One with the Dozen Lasagnas - S1-E12
Continuity mistake: At the end of the episode, when Monica scores a goal against Joey and Chandler, she is playing with the red team and scores with her strikers. In the next shot, a blue midfielder scores the goal. (00:21:05)
The One with the Fake Monica - S1-E21
Continuity mistake: In the scene when Joey and Chandler come into the cafe to say that Marcel got into San Diego zoo, the man who wants him for animal fighting turns and says that Ross is making a big mistake. At this point the monkey starts to go down Ross's arm and almost drops off but in the next shot he is far up on his shoulder again. (00:14:40)
The One With The Tea Leaves - S8-E17
Continuity mistake: When Pheobe is reading the tea leaves, Chandler lowers his cup. The camera cuts and he lowers his cup again. (00:04:30)
The One with Two Parts: Part 2 - S1-E17
Continuity mistake: When Ross and Jack are talking about becoming a father, Ross takes a drink out of a glass and puts it back down. The liquid doesn't move when put back down. (00:08:15)
The One With The Halloween Party - S8-E6
Continuity mistake: When Chandler and Ross are arguing, the nose on Chandler's costume moves. In one shot it's on his chin. The camera then cuts to Joey and Monica and back, and then it's down round his neck. (00:12:20)
The One With Monica and Chandler's Wedding, Part 1 - S7-E23
Continuity mistake: When Joey and Phoebe are talking about his co-star spitting on him, and Phoebe says that she gets spit on at work, Joey puts his hand on her arm. Then in the next shot, when Monica and Chandler are walking along, Joey puts his hand on Phoebe again. (00:09:00)
The One With Rachel's Date - S8-E5
Continuity mistake: At the end when Rachel comes into the coffee house and sees Mona with Ross, she is holding a brown paper bag. When you see her in the background, the the top of the paper bag is rolled over away from her. When the camera switches to just Rachel, the paper bag is the other way around with the top rolled over towards her. (00:21:35)
The One With The Hypnosis Tape - S3-E18
Continuity mistake: When Chandler asks for a chapstick, his hands are in his coat pockets; then Rachel asks "How are those tapes working out for you?" and his hands are clasped in front of him. (00:10:00)
The One The Morning After - S3-E16
Continuity mistake: When Ross is listening to the message from Rachel he takes a drink from a cup and sets it down, with the handle pointing toward the answer machine. In the next shot, a close-up, the handle is now towards the camera. (00:03:05)
The One With The Cooking Class - S8-E21
Continuity mistake: The night before they go to the baby furniture store, Ross and Rachel are in the kitchen talking. Just before they leave the kitchen watch Rachel's robe: in one shot the 'lapels' are neatly one over the other, and in the next shot one lapel has moved lower and looks halfway folded over as if she hastily pulled the robe together. (00:03:15)
The One Where Chandler Can't Cry - S6-E14
Continuity mistake: Jill goes to Ross' place, intending to make Rachel mad. When Ross goes to Rachel's to talk to her afterward, she rolls up her sleeves saying, "I'll give you something to think about." When Rachel is next shown, her sleeves are rolled down. Her arms also go from by her sides to crossed in front of her. (00:19:10)
The One with the Monkey - S1-E10
Continuity mistake: When Ross say 'just a smidge' after Rachel says Paulo named her breasts, Phoebe is holding tinsel and Rachel is looking at Ross. In the next shot, Rachel is looking in the opposite direction and the tinsel has vanished from Phoebe's hands. (00:06:00)
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.