The One With All The Resolutions - S5-E11
Visible crew/equipment: After his fiasco with the leather pants, powder, and lotion, when Ross is at Monica's and tells the others he's going to kiss Ben goodnight, as he starts walking to the bedroom the camera can be seen at the right side of the screen.
The One With Ross's Sandwich - S5-E9
Continuity mistake: After Joey makes up the story about how he slept with Monica once in London, in the shots facing Phoebe, Rachel, and Ross the door to Monica's apartment goes back and forth being either wide open or closed, depending on the angle of the shot.
The One With Ross' New Girlfriend - S2-E1
Visible crew/equipment: When Rachel's at the airport and freaks out after seeing Ross with Julie, right before she steps up on the chair the grey crash mat can be seen on the floor, behind the man who's wearing white pants and is seated.
The One With Ross's Inappropriate Song - S9-E7
Visible crew/equipment: When Joey jumps on Chandler to shield him from seeing Monica's sex tape, the mat they land on is visible prior to the scene.
The One Where Chandler Doesn't Like Dogs - S7-E8
Continuity mistake: In this episode, it is stated that Ross does not like ice cream because it's too cold. However, back in The One Where Ross Dates a Student, you can see Ross on a date with Elizabeth eating an ice cream.
The One With The Cheap Wedding Dress - S7-E17
Continuity mistake: Monica chooses a wedding dress at an expensive boutique, then later goes to a discount store to buy the same style dress at a half-off sale. The dress she buys is different from the one in the boutique, having wider and frillier straps. When she tries it on later at the apartment, it is now the same style as the one at the boutique. (00:04:00 - 00:07:15)
The One Where Monica Gets a Roommate - S1-E1
Continuity mistake: When Ross, Chandler and Joey are setting up Ross' furniture and Ross is bent down saying, "And I cannot feel my legs," Ross is holding up a manual. When it cuts, the manual is suddenly lying on the floor. [In other versions, such as Netflix, the first shot shows the manual falling to the floor and the second shot does not exist. However, the mistake is valid for the TV broadcast version of the episode]. (00:11:45)
The One With All The Thanksgivings - S5-E8
Continuity mistake: In the flashback where Joey sticks the turkey on his head, the subtitle says Thanksgiving 1992. However, the show started in 1994 and Joey didn't move in until the previous fall, so the earliest Thanksgiving that he was there would have been 1993. (00:04:30)
The One With The Baby Shower - S8-E20
Visible crew/equipment: Monica chases after Rachel's Mom to apologise and she falls over and you can clearly see the safety mat move when Monica falls on to it - it's blue and just down the first step. (00:22:00)
The One With The Mugging - S9-E15
Revealing mistake: When Rachel tells Joey that his agent called and he got the audition for the Broadway play, Rachel's wearing a colorful paisley top, and after Monica says, "Maybe the monkey will be at the audition," in the next shot it's Jennifer Aniston's stand-in, wearing a red plaid blouse, who is standing beside Joey. (Widescreen). (00:03:08)
The One With The Proposal, part 1 - S6-E24
Continuity mistake: In the scene where Richard is being seated next to Chandler and Monica, the amount of champagne in Chandler's glass changes. (00:09:20)
The One With Rachel's Other Sister - S9-E8
Other mistake: When Joey and Phoebe are talking while Rachel and Amy are fighting, Phoebe says the line 'Oh my God. Shouldn't we stop this?', Joey is mouthing her lines. (00:20:25)
The One with the Blackout - S1-E7
Revealing mistake: When Rachel first meets Paolo she's looking for the cat in candlelight, holding the candle in front of her. You can see her shadow on the wall, as well as the shadow of the candle, revealing that the light is not coming from the candle. (00:07:25 - 00:14:30)
The One With All The Wedding Dresses - S4-E20
Visible crew/equipment: When it's Monica's turn to catch the bouquet she and Phoebe switch places, and just as Phoebe asks her if she's ready, in the wideshot studio lighting rigs can be seen at the top of the screen.
The One Where Ross Got High - S6-E9
Visible crew/equipment: After Ross and Monica blurt out each other's secrets to their parents, when Jack says, "And we kind of figured about the porch swing," in his closeup we can see the end of the set wall, with the soundstage behind it and many tape marks on the floor.
The One Where Rachel's Sister Babysits - S10-E5
Continuity mistake: Rachel and her sister are in Rachel's room. When Rachel picks up Emma, Emma has nothing in her arms, but in the next shot when Rachel is holding her, she has a toy in her arms though she had no time to get one.
Continuity mistake: The view from Monica's apartment changes after the first few episodes.
The One With a Chick. And a Duck. - S3-E21
Continuity mistake: When Ross is asking Joey and Chandler which suit he should wear for the TV thing the brown suit is in his right hand and the blue one is in his left hand. After Ross says "really?" the shot changes to a wide view, the two suits have changed places, and the cover on the brown suit has switched to the blue one. (00:09:50)
Continuity mistake: The Chick changes breeds and gender a couple of times. It starts out as a full size White Leghorn. During the adolescent phase it is a female White Silkie. This is a bantam or small size chicken. In maturity it is a male White Leghorn.
The One With The Fertility Test - S9-E21
Continuity mistake: When Rachel takes her gift certificate to the massage parlour to claim her free massage, she hands it to the receptionist and we can see that it has been torn up into eight pieces and taped back together. Yet when Phoebe tears up the gift certificate in a previous scene, we see her tear it into only four pieces before handing it back to Rachel. (00:05:00 - 00:08:20)
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.