The One With The Boob Job - S9-E16
Revealing mistake: When Phoebe and Mike walk into Central Perk and announce they're moving in together, Chandler, Monica, and Ross, who's wearing a sweatshirt and khakis, are all sitting on the couch, and after Phoebe's comment about loading up the Volvo and wanting to be a soccer mom, it's David Schwimmer's stand-in, wearing a blue button-down shirt and jeans, who is seated beside Chandler.
The One With The Worst Best Man Ever - S4-E22
Revealing mistake: In the scene when they confront the stripper at Chandler's office. Ross starts shouting about his grandmother's ring, and as Chandler says "way to be cool, man", you can see Joey ducking behind Ross, covering his mouth with laughter. A brief cut away, then back and he's standing with the others again looking serious. Widescreen version only.
The One With The Tiny T-Shirt - S3-E19
Audio problem: At the start when Joey comes into Monica's apartment and does his little dance number, watch his mouth on his way out; it's totally out of sync with what he is saying. (00:03:15)
The One Where Monica Gets a Roommate - S1-E1
Continuity mistake: The door to Monica's bedroom is shown open, then closed, then open, and then closed during the scene in which Paul arrives for their date. (00:09:00)
The One After Joey And Rachel Kiss - S10-E1
Factual error: The main part of the episode is played out in Chandler & Monica's hotel room (which is situated in between Rachel's and Ross's rooms). The window is opposite the audience and the door on the right. However whenever we see inside Ross's hotel room, he has a window opposite the audience as well. This is impossible as this should be where the hallway is to get to the main door to Chandler & Monica's room. Also, throughout the episode Monica, Chandler & Phoebe all listen through the walls (on either side of their room) to what the other friends are saying, meaning there must be no gap between Ross's and Chandler & Monica's rooms- making the window in his room geographically impossible.
The One With The Red Sweater - S8-E2
Plot hole: They say that whoever the sweater belongs to left it behind, but in The One Where Rachel Finds Out that Ross videotaped them together it is clear that Ross is not wearing anything under the sweater, therefore for Ross to have left it behind he must have gone home topless.
The One Where Monica Gets a Roommate - S1-E1
Continuity mistake: When Monica is saying bye to Paul, Paul goes to the screen left but the stairs are at the screen right. (00:19:20)
The One Where Monica Gets a Roommate - S1-E1
Continuity mistake: In the scene where the friends scoot the table over to see Monica's date with "Paul the wine guy", the cup in front of Chandler changes position. (00:18:55)
The One Where Dr. Ramoray Dies - S2-E18
Continuity mistake: When Monica is trying to cheer Joey up, she tells him that she straightened out his shower curtain so it wouldn't get mildew. But in a previous episode (The One Where Eddie Moves In), Joey gives the gang a tour of his new apartment; the shower is clearly visible in the bathroom, and it has a door. (00:18:10)
Deliberate mistake: 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 Chicken Pox - S2-E23
Factual error: The symptoms (spots, fever, etc) of chicken pox only develop 2 - 3 weeks after exposure to the virus. Therefore, when Ross knew that Ben had chicken pox, Phoebe shouldn't have developed the condition until at least two weeks later, and Ryan certainly shouldn't have developed the disease until the end of his stay (as Phoebe was his only contact with the virus). (00:09:30)
The One With Joey's Porsche - S6-E5
Other mistake: Immediately after Joey walks in as Chandler is handling the baby, we see him staring and waving in a direction past the stage. (00:06:55)
The One with the Sonogram at the End - S1-E2
Factual error: The sonogram shows a fetus that's much too far-developed for Carol's pregnancy. That fetus is at least 20 weeks along. (00:22:00)
The One With Ross's Denial - S6-E3
Visible crew/equipment: After Ross tells Rachel about Warren's apartment he gives her Warren's phone number, and in the closeup while Rachel hugs Ross, and also in the following wideshots which face Ross the camera can be seen at the right side of the screen.
Continuity mistake: Chandler and Monica's wedding day is May 15th 2001, as mentioned on the show. In Series 9, some time in January 2003 (as it is after the characters have celebrated Christmas and New Year), it is established that Rachel has been on maternity leave for five months, which means she took her leave in August. Since she realised she was pregnant in May the year before, when Monica and Chandler got married, this means her pregnancy lasted from April/May 2001 to August/September 2002, a time span of 16 to 17 months. This cannot be explained by saying that episodes don't have to take place in the month they air, as we are given the dates by the characters on the show.
The One With Monica and Chandler's Wedding, Part 2 - S7-E24
Revealing mistake: At the point in the ceremony where Joey forgets the rings, the camera pans out and you see Chandler step sidewards to get the ring from Ross, and Monica step sidewards to get the ring from Rachel and Phoebe. As Monica moves, her dress slides across and one of her feet is revealed; it has what looks like a brown boot or heavy brown shoe on it. (00:20:05)
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 Joey's Big Break - S5-E22
Factual error: Joey and Chandler are supposedly crossing the George Washington Bridge but the exterior background viewed from inside the car is very obviously a repeat reel of another bridge, and their conversation goes on for far too long for the actual crossing. And the cutaway shot after Chandler is kicked out of the car shows an aerial view of the Brooklyn Bridge.
The One With the Sharks - S9-E4
Continuity mistake: In "The One With the Sharks", Phoebe talks to Joey about how he's never been in a long term relationship. He has been, though - Angela Delveccio, who he went out with for three years. (00:21:40)
The One with the Blackout - S1-E7
Revealing mistake: When Chandler is choking on the gum, you can see it is still in the left side of his mouth in a huge lump. (00:16:45)
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.