Corrected entry: When Monica and Chandler walk into the dance hall, a cameraman is seen following them.
The One Where Ross and Rachel...You Know - S2-E15
Corrected entry: When Joey and Chandler are ordering a pizza they tell the pizza place to send the food to apartment 20 which is Monica and Rachel's apartment but it is clear that Monica isn't home because she is catering and Rachel is on her date with Ross, so how do they get the pizza.
The One With The Ring - S6-E23
Corrected entry: When Phoebe accidentally allows Chandler's ring to be sold, she tells him that she overheard which restaurant the guy said he was taking his girlfriend to, so they can go find him and persuade him to exchange rings. However, in the previous scene we see that Phoebe was paying no attention at all to that guy - she was too busy trying on jewellery. She wasn't even within earshot of the guy, and we see from her reaction that she hadn't even noticed him at all before he left the store with the ring (during which time he doesn't say anything at all). (00:16:50)
Correction: She may have been trying on all the jewellery, but she would have certainly heard the guy say he was going to a certain restaurant with his girlfriend before he left. Obviously she then realised the ring the man bought was the one Chandler had his eye on so reacted.
The One With The Giant Poking Device - S3-E8
Corrected entry: When Monica and Rachel are banging their heads against the wall, you can see someone in the background by the bathroom when Rachel walks away towards the living room in pain while saying, "if it isn't a headboard, it isn't worth it."
Correction: That is Phoebe talking on the phone.
The One with the Dozen Lasagnas - S1-E12
Corrected entry: At the end, Phoebe and Monica are sitting at the table eating the meat lasagna. Phoebe would not eat this as she is a vegetarian. (00:21:35)
Correction: Early on in the episode, Monica suggested vegetarians can scrape. It is possible that Phoebe is doing exactly that. Also, two episodes later ("The One with the Candy Hearts"), it is revealed that Phoebe, in fact, does enjoy meat, as long as no-one tells her it is meat she is eating. She is poor and has never been able to afford meat.
The One With Five Steaks and an Eggplant - S2-E5
Corrected entry: When the whole group is eating dinner, Ross miscalculates what everyone owes. At first, he states "Everyone owes 28 bucks." He assumes all six people will chip in, so he was starting from a total of $168. Ross then decides, "Monica shouldn't have to pay" (since it's her birthday), so he instead divides the total by five to get each person's individual amount owed. He can be heard saying, "So... five of us is... $33.50 a piece." $33.50 is incorrect. He should have arrived at a figure of $33.60, ten cents higher per person, because $168 divided by 5 is $33.60. (00:10:05)
Correction: So Ross rounded to the nearest half-dollar. That's not a mistake.
The One With Rachel's Other Sister - S9-E8
Revealing mistake: When Chandler grabs his bag he accidentally lets the box full of wedding china fall off the stool. Watch closely (play in slow-motion). The box falls on the floor and shattered china falls out. The china was ALREADY broken before it falls on the ground. (00:23:00)
Corrected entry: The first version of Central Perk's logo, seen from season 1, presents a perspective error on the right top corner of the left cup of coffee. Some seasons later, the logo receives a tiny correction in the same area, that makes the perspective "OK" to the eye. The cup line straightens up, and the steam is brought a little closer to the ribbon containing the name.
The One Where Eddie Won't Go - S2-E19
Corrected entry: Rachel says that Phoebe slept with Jason Hurley one hour after they broke up. Monica seems surprised by this even though she already found out that Phoebe slept with him right after he and Monica broke up in "TOW George Stephanopalous". (00:14:30)
Correction: The understanding is misconstrued. Monica knew that Phoebe had slept with him (ergo The One with George Stephanopoulos); the surprise comes from the fact that she had slept with him only one hour following their breakup. This is not a continuity mistake but a misunderstanding of meaning.
The One Where Rachel Quits - S3-E10
Continuity mistake: At the start of the scene in which Ross breaks the little girl's leg, we can see that the stairs are just separated by railings. Later in that scene, when Ross and Chandler start walking up the stairs, there are walls on both sides of them. (00:01:40)
Suggested correction: This isn't completely true. In the first shot, we see railing in the middle, as shown in the screenshot. Then, the shot changes to upstairs, looking down, and we can see the wall, as shown in the bottom picture. However, at the end of the wall, it goes into the railings, which is visible just as the pair start to climb the stairs. A screenshot of this can be seen here: https://ibb.co/bmZ8xTc, making it easier to understand.
The problem with the screenshot is the right side is circled to show there's no railing, but the railing is on the left side (to the character's right), but Chandler is blocking the view of the railing. Ssiscool's screenshot is before Chandler starts to walk up the stairs, which is why it's seen.
The One With Monica and Chandler's Wedding, Part 1 - S7-E23
Continuity mistake: When Monica and Chandler walk toward the middle of the room the camera cuts to Ross talking to his parents saying that he could kick Chandler's ass, Behind them are Monica and Chandler in a different place, listening and laughing to Ross. (00:10:50)
Suggested correction: The shot is continuous. There is no continuity mistake.
The One With The Embryos - S4-E12
Corrected entry: When the Gynaecologist says we insert 5 embryos for a 25% chance one will attach. Wouldn't one in five be a 20% chance?
Correction: That's not how the statistics work. There's no guarantee that any embryos will attach at all - presumably the odds of any one embryo attaching is 5%, so adding 5 makes the odds 25%. It has to be a balance between ensuring a vaguely decent chance of a viable pregnancy vs. the risk of having multiple babies. For example if they inserted 20 embryos to potentially "guarantee" at least one would attach, at least statistically, the odds would be much much higher of say 5 of them attaching, which would cause great complications for the mother and indeed the fetuses, not all of which would likely survive.
The One Where Heckles Dies - S2-E3
Corrected entry: When Mr. Heckles goes back to his apartment, it looks as if he is going upstairs, when actually his apartment is below Monica and Rachel's.
Correction: When Heckles turns away from Monica's doorway he steps up onto the upper landing, not the stairs. The stairwell is at the end of the hallway (on this upper landing) where Heckles is heading, and we don't see him reach the stairs. Other episodes such as S8xE7 "The One With The Stain" and S10xE8 "The One With The Late Thanksgiving" show the hallway.
Factual error: This is a mistake for two of Ross' three marriages. Ross wasn't even actually "married" either time. With Emily, he goes through the ceremony of the marriage and then Emily immediately disappears. You're not actually married until the certificate is signed and sent in, with the signatures of the witnesses. Which was not happening after that disaster of a wedding. Nor were there any witnesses filling out and sending in certificates after the drunk Vegas wedding either.
Suggested correction: The certificate of marriage is signed and sent in by the person performing the ceremony, and he asked if they wanted to continue with the marriage and it was completed. Secondly the marriage to Rachel they discuss how they paid witnesses to the wedding. I believe they mention Elvis?
Suggested correction: At the Vegas wedding, the chapel likely provided witnesses who took care of the paperwork.
The One Where Underdog Gets Away - S1-E9
Corrected entry: When Monica yells about how many potatoes she made when they were locked out, she calls Joey Mario. Not Joey.
Correction: Because of Joey's VD poster: "What Mario isn't telling you".
The One With Joey's Porsche - S6-E5
Revealing mistake: When Phoebe knocks over the vase she glances at it just before throwing the diaper. (00:15:10)
Suggested correction: The bin for the diapers is near the vase, and she glances at that before throwing the diaper.
The One with the Blackout - S1-E7
Audio problem: When Phoebe is counting down to the last candle burning out you can see Joey blowing out the candle, but the sound of his blow is only heard a second later after he sits back. (00:22:50)
The One with the Thumb - S1-E3
Continuity mistake: Monica's snort, Joey's knuckle cracking, and Phoebe's hair chewing are never seen in any other episode, before or after this one.
The One With The Ball - S5-E21
Corrected entry: While Sphynx cats are not generally hypoallergenic, Monica could not have had an allergic reaction because her exposure to the cat was minimal and she didn't touch it.
Correction: Many people, like my mother, are highly allergic to a cat's dander or saliva and will have a reaction just being in a room where the cat had been. No actual contact is needed in order to have a reaction. The cat doesn't even need to be there at the same time as the allergic person. Monica could easily have still had an allergic reaction to the cat eve though her exposure was minimal and she didn't touch it.
The One With The Red Sweater - S8-E2
Corrected entry: Ross and Chandler are in the ballroom looking for the disposable cameras. Chandler comes out from under the table, a disposable camera is in clear shot on top of the table but neither character notices.
Correction: This is an actor portraying a wedding videographer.