Continuity mistake: In season 1 ("The One with George Stephanopoulos") the date's October 20th, and Ross says his birthday "was 7 months ago". In season 4 ("The One With Joey's New Girlfriend") Gunther is pretending to collect birthdays, and Ross starts saying "mine's December the..." before being cut off. In season 9 ("The One Where Emma Cries") Ross says his is on October 18th. (00:15:10)
Continuity mistake: Across Series 3 - 5, Ross remains the same age (29) for all three years. In Series 3 he tells Chandler, "No thanks, I'm 29" when offered chocolate milk; in Series 4 when the guys try to party without Gandalf they all say they're 29; and in Series 5 when he's trying to reconcile with Emily, he says either he does, or he would get divorced for the second time before age 30.
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.
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.
Factual error: Phoebe's greatest love David leaves her to go to Minsk and comes and goes throughout the whole show, telling stories of Minsk on the way i.e. of the long months when the sun does not rise and the constant freezing cold. Minsk, being the capital of Belarus (although claimed to be in Russia for reasons explained in Corrections), is approximately as high up north as i.e. Wales and northern coast of Germany, and even during the winter months there is always daylight for several hours. Although winters in Eastern Europe are cold, the kind of coldness described by David would more believably happen in Siberia than Minsk, and the summers are just as nice and hot as everywhere in Europe.
Continuity mistake: The view from Monica's apartment changes after the first few episodes.
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.
Audio problem: Any scenes outside Central Perk always sound as if they're on a set (which let's be honest, they are) - the sound is often muted and footsteps on the pavement have a very slight echo to them.
Continuity mistake: Ross always had an end apartment. At some point in Season 8, his apartment became a middle apartment. In earlier season, when Joey had mistaken Ross' for the hot girl's apartment, it showed the apt as an end apt. In TOW Joey tells Rachel, when Joey asked Rachel to have dinner at Ross' apt, it showed another hallway where Joey came from which is supposed to be the wall/ Ross' kitchen.
Other mistake: In 'The One with the Ick Factor' (Season One) Monica states that she is 26 years old. This episode takes place in 1994/95. However there are numerous other episodes such as 'The One With All The Thanksgivings' that show Monica and Rachel (who are the same age and in the same class) still being in High School in 1988. In 'The One Where The Stripper Cries' the two have to sneak into a college party in 1987 as they are still high school age. They should be at least 19 or 20 in 1987/88.
Continuity mistake: In addition to Ross remaining 29 for several years, he also gets younger as the years go by. When he marries Emily he is thirty (Monica describes herself as "a single mother with a 30 year old son"), but when he divorces her he is twenty-nine.
Continuity mistake: In the early episodes there is a wooden post in front of the door at Monica's apartment. Remember when Monica and Rachel were looking after Ben and Monica bangs his head - Monica shouts "Monica bang" and runs into it. The post doesn't appear in later series. This was due to the post being an obstacle to shoot around.
Character mistake: In some episodes the facade of Central Perk is shown. On the window the word "cappuccino" is misspelled, it says "cappucino."
Continuity mistake: Apparently Ross is Monica's older brother but in "The one where they're going to party", Joey states that the three men are all the same age ("we're 29, we're not women"), and in The One with the Birth Monica says she is 26 and Joey says he is 25. If Joey is younger than Monica, and Ross and Joey are the same age, then there's definitely a mistake somewhere.
Other mistake: In his apartment from later episodes, Ross' kitchen door is on the same wall as the front door, and it is a long and narrow kitchen, but Ross' apartment is between two others. The floor plan doesn't make sense unless the kitchen is only a foot wide, otherwise it would jut off into the hallway.
Other mistake: In almost every season when the friends are in the coffee shop, it shows a front view of Central Perk; and a lot of times, in different episodes, the same (unaging) lady with blonde curly hair is walking past the shop. There are also a couple of different episodes where two babies with red hair are being carried past the coffee shop.
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.
Continuity mistake: The bathroom in Chandler and Joey's apartment constantly changes in the direction it opens in. Sometimes the doorknob is on the left, sometimes it is on the right. The changes occur throughout the seasons, and in some cases the changes occur in back-to-back episodes.
Visible crew/equipment: Most of the time when they show the Central Perk logo on the window. If you look inside you see the set because the wall doesn't go far enough.
Continuity mistake: The number of the flats near the start of the first series was 4 and 5. After a few shows they changed to 19 and 20. [The producers realised that the numbers 4 and 5 were for apartments on the lower floors and had shown Monica's apartment to be much higher up so they changed the numbers - it's still a mistake but there's the reason.]