The One Where Paul's The Man - S6-E22
Continuity mistake: When Rachel and Paul are on the couch at the cabin, the diet coke Rachel is holding switches (repeatedly throughout the scene) from her left hand to her right. (00:12:00)
The One With The Tiny T-Shirt - S3-E19
Continuity mistake: When Joey says "Pete Chicago", there is a green cup on the table next to him; in the shot immediately after that, he is holding it down in front of himself. (00:09:35)
The One With The Truth About London - S7-E16
Continuity mistake: In the opening credits, Monica and Chandler are talking about which minister to use. Chandler's feet are crossed and on the table; at first his left leg is over his right, then in another shot his right leg is over his left. (00:02:45)
The One Where They're Going To PARTY! - S4-E9
Revealing mistake: When Phoebe is looking at the front page of the "Chelsea Reporter," Monica says her restaurant review is on the back page. However, Phoebe doesn't turn to the back page, but merely flips the paper over, looking at the bottom of the front page (and finding the review there). (00:07:30)
The One Where No One Proposes - S9-E1
Continuity mistake: In the one where Rachel is in the hospital after having Emma she is in the bed feeding Emma and Ross is sitting right beside of her. They are talking about their relationship and what should come. If you will watch each shot as they go to showing Rachel's face then Ross's you can see that Rachel's hair in one shot is in front of her shoulder and the next shot it is behind her shoulder. This happens several times. (00:20:55)
The One With The Rumor - S8-E9
Continuity mistake: In the episode with Brad Pitt in it, when Ross and Will are standing by the fridge and Rachel comes in, Will's right hand jumps around after he crushes the decoration on the wall. (00:08:40)
The One With The Cake - S10-E4
Plot hole: Judy says that both her parents died very young. However, we saw her mother's death in "The One Where Nana Dies Twice" and she was quite old. Also, in Season 7, 'TOW Phoebe's cookies', Jack Geller tells Chandler that his father in law still thinks he is a lawyer and says "So if you ever see me giving them legal advice, just nod along," implying that Judy's parents were both still alive.
The One Where Rachel is Late - S8-E22
Plot hole: The times in this episode vary a lot. Monica and Phoebe first bet about Rachel giving birth, then it goes to the scene with Joey and Chandler at the movies. It then returns to Monica and Phoebe the next day with Monica losing the bet. However, a little while later, it goes back a day and shows Joey and Chandler at the movie premiere again. (00:09:00)
The One with Two Parts: Part 1 - S1-E16
Continuity mistake: At the start of the episode, Chandler is holding onto a menu with his right hand. In the next shot, it is gone. (00:00:10)
The One With The Inappropriate Sister - S5-E10
Visible crew/equipment: At the start as Joey comes out, watch the tv and you will see a line of stage lights. (00:00:05)
The One With Joey's Dirty Day - S4-E14
Continuity mistake: When Rachel is asking Ross if he will take Emily to the opera, the amount of biscuit in his hand changes throughout. (00:06:15)
The One With The Screamer - S3-E22
Continuity mistake: When Gunther brings Chandler the cup of coffee from the woman at the bar, Chandler does not take it by the handle; however the shot quickly switches to him holding it by the handle. (00:14:00)
The One With The Video Tape - S8-E4
Continuity mistake: When Rachel has the tape and is explaining why she wants to watch it, she is holding it in her left hand with her right hand on her chest, when the shot changes to behind her, she has it in her right hand. (00:15:30)
The One With The Hypnosis Tape - S3-E18
Continuity mistake: In the diner scene, Rachel makes the comment about working in fashion and straight men, and her left hand is empty; in a flash, a piece of bread appears without her picking it up. (00:06:40)
The One With The Truth About London - S7-E16
Continuity mistake: After Rachel shows Ben the pencil and quarter prank, she lays the pencil down and it is pointing straight ahead. When the pencil is shown again it has moved. (00:05:45)
The One With All The Thanksgivings - S5-E8
Visible crew/equipment: Just as Monica finishes her story and Chandler is shown, the reflection of the stage lights is visible in the TV. (00:18:20)
The One With a Chick. And a Duck. - S3-E21
Revealing mistake: When Joey comes in with the shoebox containing the chick, it looks like a normal sized box, about 5 - 6 inches deep. But when Joey takes the lid off, we can see that the chick must be elevated on something inside the box, since we can see its entire body. Surely, the chick can't be more than 6 inches tall? (00:05:05)
The One Where Rachel Goes Back To Work - S9-E11
Continuity mistake: Ross bends over Emma to kiss her and gets the bow stuck on his head. The bow is slightly over his left eye but in the next shot, it has moved to the centre of his forehead. (00:11:35)
Continuity mistake: When Ross and Rachel are in bed, and Rachel says she remembers laughing a lot, and Ross says "Then we didn't have sex," Rachel turns her head away from Ross twice. (00:03:10)
The One With Phoebe's Ex-Partner - S3-E14
Visible crew/equipment: Just after Chandler says "I'm the smallest person in the world", and Joey comes out of his room, check the top left of the screen - you can see a boom mic track Matt LeBlanc across the set, and the bottom of some stage lights in the middle. (00:14:40)
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.