Other mistake: When Phoebe breaks into the other Phoebe's house, after she mentions that the man on the picture is "Chuck Mangionne" the door behind her opens by itself. (00:19:40)
The One In Barbados, part 2 - S9-E24
Other mistake: The layout of the hotel surrounding the main cast's rooms as displayed during the sequence where Monica, Chandler, and Phoebe eavesdrop on Ross, Charlie, Rachael, and Joey is physically impossible. Ross enters the room Monica, Chandler, and Phoebe are in through the same wall they are using to listen in on Charlie and him, though while the camera is in Ross and Charlie's room, no door can be seen in that wall. Furthermore, a hallway can be seen through the doorway while Ross holds the door open, implying that the two rooms do not share a wall in the first place.
The One With The Girl From Poughkeepsie - S4-E10
Factual error: Ross falls asleep on the train and ends up in Montreal, where the woman he meets tells him "it's only a 2-hour ferry ride to Nova Scotia." Yeah, more like a journey of between 13 hours (12-hour car ride plus 1-hour ferry) and 23 hours (multiple bus legs plus 1-hour ferry.) The character could have been lying to Ross, but it's not presented that way at all.
The One with the Thumb - S1-E3
Factual error: The show is set in New York. Joey was a cast member on Days of Our Lives, which is filmed in Los Angeles, but is still always around.
The One with the East German Laundry Detergent - S1-E5
Character mistake: In the laundromat, there is a sign where the word clothes is misspelt cloths. (00:16:00)
The One With The Truth About London - S7-E16
Audio problem: During Rachel's scene sitting on the couch with Ben talking about her and Ross and her sisters, Rachel's mouth moves in any scene showing Ben, but without her being heard.
The One With The Video Tape - S8-E4
Audio problem: When Joey says "this is not fair to her, let's just forget the tape", his mouth stops moving after he says "forget", making it look like the audio is coming from nowhere. (00:12:07)
The One With Monica's Thunder - S7-E1
Audio problem: When Joey walks out of the bedroom dressed as a 19 year old, when Chandler says the line "Yes, on a scale from 1 to 10, 10 being the dumbest a person can look, you are definitely 19", you can see that the audio doesn't match the words. It looks as though Matthew Perry says "not 19" and they had to re-dub over post production with "19." Subtle, but it's there. (00:08:50)
Suggested correction: Just watched it twice and seems to be fine... Can't see any issue.
The One With All The Wedding Dresses - S4-E20
Continuity mistake: Rachel takes a drink from the Jones Soda and sets the bottle down. A second later, the bottle is fuller than it was to begin with. (00:10:05)
The One After Ross Says Rachel - S5-E1
Audio problem: Monica tells Ross that he should keep looking for Emily, and Chandler says "Yeah, for about 30 minutes." But the audio doesn't match up with his lip movements, indicating this line was dubbed. (00:10:26)
The One Where Monica Gets a Roommate - S1-E1
Other mistake: Ross is a paleontologist but is seen working with the display on early humans which would be more of a job for an anthropologist.
The One The Morning After - S3-E16
Visible crew/equipment: When Phoebe says they've been quite for a long time, a boom mic is visible above her (only visible in fullscreen version). (00:21:40)
The One With The Proposal, part 2 - S6-E25
Audio problem: In the pizza place, after Chandler tells Monica to "put down [her] copy of The Rules..." you hear Monica say, "You know what. I gotta go" when her lips aren't moving. (00:04:26)
Other mistake: When Joey sat in the paint the first time, there was a mark, but the second time, it didn't leave a mark.
The One Where No One's Ready - S3-E2
Visible crew/equipment: When Ross tells Chandler and Joey, "I'd feel a whole lot better if you got dressed now," the reflections on the TV screen show there's a crew member at the side of the sofa wearing a white T-shirt, as well as the camera and camera operator behind the sofa. (00:00:30)
The One Where No One's Ready - S3-E2
Visible crew/equipment: After Rachel agrees that it will indeed take her 6 or 7 minutes to get dressed, with the caveat, "Once I figure out what I'm wearing," it cuts to Joey offering Ross the "glass of fat" as he stands behind the couch. In the next shot of Ross, the reflections on the TV screen show Joey is still sitting on the couch, and the camera and camera operator (wearing a dark shirt and raises an arm) behind the sofa. (00:01:15)
The One Where Joey Dates Rachel - S8-E12
Factual error: The premise of Chandler filling the high score list with dirty words is not possible for this game. Ms. Pac-Man only keeps track of a single high score, which is always displayed at the top of the screen. Unlike many other games of the era, there is no screen showing a list of additional top scores alongside initials during the attract mode, and there is not even a mechanism for entering initials within the game's code.
The One With The Bullies - S2-E21
Visible crew/equipment: When Chandler falls behind the couch, the mat he lands on is visible.
The One With The Tiny T-Shirt - S3-E19
Audio problem: When Joey chants "Oh Mommy, Oh Daddy, I am a big ol' baddie" his voice doesn't match his lip movements.
The One with George Stephanopoulos - S1-E4
Continuity mistake: When Rachel is explaining that she's not going back home, the friend with her arm on the back of the sofa behind her has her sleeve part the way up towards her elbow leaving a very prominent gap between the sleeve and bracelet. When the camera comes back to her the gap has now gone. (00:08:20)
Suggested correction: It's presented as sarcasm. Not a mistake.
Ssiscool ★
Not clear on where you got that it's presented as sarcasm; nothing in her delivery says so.