The One Where Ross and Rachel...You Know - S2-E15
Visible crew/equipment: At the start, when Chandler and Joey sink into their new Barcaloungers and proceed to recline, the camera can be seen at the right side of the screen. This can also be seen in the show's intro in later episodes. Additionally, set lighting was reflected on the TV screen earlier when Chandler had mentioned "Green Acres."
The One With All The Rugby - S4-E15
Factual error: Rugby starts with a kickoff and not with a scrum, only the forwards on the team participate in the scrum not the whole team, and there is no scrum half to retrieve the ball from the scrum. The scrum took far too long. There is no structure to the scrum like there should be (props, locks, flankers etc). He doesn't seem to have a team and for some reason everyone just jumps on him, there is no line with one team and another it just seems like a free for all which is nothing like rugby, rugby halves are ended when the ball goes out of play which does not happen here - he just decides to end it for no real reason. Several fouls are committed in the ruck such as coming in from the sides, diving off their feet and holding onto the ball which aren't being penalised. You're not allowed to kick people or harm them intentionally in rugby, there are no cups in rugby but there are mouth guards, which no one is wearing.
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 With Chandler's Dad - S7-E22
Revealing mistake: In the shot of the highway with Ross and Rachel in the Porsche before the police officer walks to them, it is very obvious that the people inside the car are not actually them.
The One With The Prom Video - S2-E14
Visible crew/equipment: After Chandler throws the basketball and breaks the lamp, in the next shot of Joey as he mentions the bracelet, we can see the reflection of a crewmember (standing behind the foosball table) on the left side of the TV screen (Chandler's reflection is at the center of the screen).
The One Where Ross and Rachel...You Know - S2-E15
Visible crew/equipment: While "The Dick Van Dyke Show" is playing on TV, when Rachel walks in and heads over to Ross, a crewmember can be seen at the left side of the screen, behind Phoebe.
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 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 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 Stain - S8-E7
Visible crew/equipment: When Brenda is cleaning the top of the cabinets, stage lights can be seen on the top right-hand corner of the screen. (00:11:45)
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 All The Jealousy - S3-E12
Visible crew/equipment: After the barbershop quartet sing their "Ross song" to Rachel at her office, at the start of the next shot the camera can be seen at the right side of the screen, just as Ross walks out of Rachel's bedroom.
The One Where Chandler Can't Remember Which Sister - S3-E11
Visible crew/equipment: When Phoebe and everyone else hear the guy upstairs with the "pogo stick" Joey gets up to give him the "butt kicking of a lifetime," and just as Joey walks around the couch we can see a crewmember and camera, at the left side of the screen.
The One With The Rumor - S8-E9
Continuity mistake: When Ross hands Rachel the Yams in order to break the tension, the amount of food on both Ross' and Will's plates changes between shots. Also several wine glasses appear on the table. (00:12:30)
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 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.
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.