The One With Rachel's Assistant - S7-E4
Visible crew/equipment: When Rachel holds up the Polaroid she took of her assistant, it appears to have been one of the actual photos taken in the earlier scene; you can see the edge of the set behind him, and the overhead lights in the black space to the right of the photo. (00:07:40)
Continuity mistake: Right at the end when Ross hits Chandler with his rolled up magazine, he only folds over one-third of the magazine onto the middle third. When it cuts to him hitting Chandler (without a time delay between him hitting Chandler and folding the magazine) the magazine is fully folded over. (00:21:30)
The One With Joey's Dirty Day - S4-E14
Continuity mistake: When Rachel is telling Monica and Phoebe about her night out with Joshua the previous evening, Monica is holding a glass of orange juice. To begin with the glass is half full, but then changes to being almost full to the top, and then back to half full. (00:09:00)
The One With The Cooking Class - S8-E21
Revealing mistake: When Ross confronts Rachel saying are you sure you don't want more, she gets up saying she has to pee - as she stands up, she turns her back to the camera, and you can she the buckle for her mum-tum on her back. (00:20:35)
The One With All The Thanksgivings - S5-E8
Continuity mistake: In the scene where they take Chandler to the hospital, Monica hands the bag of ice containing Chandler's toe to the doctor. The doctor is wearing gloves in one shot then no gloves in the next. This goes back and forth three or four times. (00:07:45)
The One With Ross's Wedding, part 2 - S4-E24
Continuity mistake: When Phoebe is on the phone to Emily's mum she is making a sandwich. We see her put mayonnaise on it, the shot flicks to Emily's mum then comes back to Phoebe and the mayonnaise is now gone. (00:26:45)
The One Where Rachel is Late - S8-E22
Continuity mistake: In one scene, Rachel is using the bathroom. She yells out to Monica and Phoebe "Whose turn is it to help me get up?" A few scenes later Rachel goes to the bathroom again, but gets up with no one helping her. (00:07:05)
The One With The Screamer - S3-E22
Continuity mistake: When Tommy starts shouting at the duck it changes its position. (00:15:15 - 00:18:00)
The One With Rachel's Date - S8-E5
Visible crew/equipment: When Chandler tells Bob "it's his fault", there is a bin behind him that reflects the stage lights. (00:16:10)
The One With The Cake - S10-E4
Revealing mistake: When Rachel and Ross are on the way back, if you look at Ross you can see a purple tinge to the side of his face. This is from projecting the background. (00:17:30)
The One With The Blind Dates - S9-E14
Continuity mistake: When Phoebe and Joey are eating the baby food, Phoebe says 'Maybe once they start dating and see what's out there..." she puts the spoon in her mouth. Then when the shot changes to show Joey, her spoon is back in the jar. (00:03:25)
The One With Joey's Award - S7-E18
Continuity mistake: When Joey sets down the award he sets it at a straight angle to the counter edge, but when Joey catches Rachel about to pick it up it is angled and not straight. (00:13:45)
The One With The Dollhouse - S3-E20
Continuity mistake: When Chandler's in Rachel's office, she tells him the lingerie isn't kept there. He says, "Yes I realise that", and starts reaching for the catalogue she's about to give him. Camera cuts, and he's now standing with his hands in his pockets, reaching out again when she gives him the catalogue. (00:02:40)
Factual error: In season 4 generally, Phoebe has the shortest pregnancy ever. She gets pregnant as the boys win the girls' apartment. In the next episode Ross meets Emily, after 6 weeks he gets engaged to Emily and they then choose to get married 4 weeks later. Phoebe would only be 10-12 weeks pregnant by the time they fly to London, not in her third trimester.
The One Where Joey Dates Rachel - S8-E12
Continuity mistake: After Chandler has been playing the Ms Pacman machine all day he claims his hand has seized up and it appears to look like a claw. From then on in that scene, his hand manages to move around fine. It goes in his pocket, and he waves it around like normal, and then when he mentions his hand again at the end of the scene, it appears to have seized and is clawlike again. (00:10:30)
The One With The Embryos - S4-E12
Continuity mistake: When the friends are talking about the contest to prove who knows each other better, Monica says to Joey "Are you scared?" and Rachel has her arms crossed. Joey says "No" and in the next shot of Rachel she is crossing them again. (00:05:00)
The One With Joey's Porsche - S6-E5
Factual error: Phoebe discovers that one of the babies has wandered away, and finds the baby inside the open drawer. Yeah, no. These babies aren't even walking yet, much less are they able to lift themselves up past the height of their own bodies and launch themselves into a drawer.
The One With The Girl Who Hits Joey - S5-E15
Other mistake: When Ross and Phoebe are eating cheese puffs and party mix out of the bags, Phoebe's bag reads "Heez-I" instead of Cheez-It, and the brand "Sunshine" has been covered.
The One With Phoebe's Birthday Dinner - S9-E5
Factual error: The restaurant refuses to take anyone's orders until they're all seated, which means that the group is sitting, occupying a table, without earning any revenue for the restaurant. There isn't a restaurant in NYC that would have such a policy, since it basically guarantees that a table becomes a revenue vacuum. (It also makes for very unhappy customers, but that doesn't qualify it as a factual error, since some restaurants might be actually stupid enough to not care about their return customers).
The One Where Joey Speaks French - S10-E13
Other mistake: When Chandler, Monica and Erika arrive back at the apartment after sightseeing, Chandler enters the apartment from the left (past where their apartment door is) rather than from the right, as if he came up the stairs. He obviously is entering from the stage and didn't take the time or cue to appear to enter from the direction of the stairs.
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.