The balcony that they often hang out on, does not match the exterior shown of the building where they live.
In an early episode, it is revealed that the first (and at the time, only) person Ross ever slept with was his first wife Carol. However, several years into the show, there is an episode in which Chandler says that Ross slept with the woman who cleaned their dorm building while they were in college.
In the earlier episodes the balcony of Monica's flat is nearly square shaped but in the later episodes it is more rectangular and a lot smaller.
In the initial episodes, Monica's apartment number is 5 and Joey/Chandler's is 4. In the later seasons Monica's apartment number becomes 20 and Joey/Chandler's is 19.
Rachel's last name is spelled "Green", as seen in The One with the Cake (2003), on the cake box is written "Green", such as it appears in Warner Bros. website and in the series credits, but in her office door at Ralph Lauren it is erroneously spelled "Greene", as well as on her invitation to Ross and Emily's wedding.
Ross is a paleontologist but is always seen working with the display on early humans which would be more of a job for an anthropologist.
Ross says the Bapstein-King Comet Shower technically began 7 billion years ago. Technically, that's back to around 3 billion years before the Earth formed.
Throughout the series, visitors show up to the characters' apartments without the characters expecting them. None of them live in a building with a doorman, so visitors would have to be let into the building by the people living there.
The Hawaiian word for cigarettes is not "Lailonailukoos", it is "Puhi paka, kikaliki".
Even the show's designer has commented on the fact that the Central Perk sign on the window can be read correctly from both inside and outside the coffee shop. But viewers have noticed that the logo is symmetrical and suggest that it isn't unfeasible for the owners to have placed a sign on the inside of the window which completely covers the one on the outside, making it readable from both sides.
Joey is a regular on "Days of Our Lives" while living in New York. But "Days of Our Lives" is taped in Los Angeles. However, since most characters featured on the fictitious "DOOL" were not actually on the real show, the fact that it's filmed in New York could just be another characteristic to this fictitious version.
In the final episodes when Rachel gets ready to leave the country for Paris for her new job she says how she worked for Ralph Lauren for 10 years, but she didn't start that job until 1999 and the show ended in 2004, therefore she couldn't have worked there for 10 years. Though five years have passed for the viewing audience, it's possible that 10 years have passed in the Friends universe in that time span.
In the one where Ben auditions for a soup commercial, the LEGO tower he builds in the waiting room changes colours from shot to shot.
Several times throughout the series the cast can be seen with Safeway Select products, which are available in California where the show is taped but not in New York where the show takes place.
There is a typo on the sign in the Laundromat. It says the management will take no responsibility for lost or damaged "cloths".
Due to optical refraction on the candles, when Rachel hugs Ross in London it looks like the back of his suit is on fire.
The mixed tape chandler makes for Monica has Janice saying "happy birthday" at first but later she sings "MY Valentine"
Rachel's birthday: In episode 4x05, she says she was born on May the 5th (Taurus) but later, in 7x22, when a policeman asks for her driver's license, he looks at her birth date and remarks that she is an Aquarius (January/February).
Ross' birthday changes several times during the show. In series 1, he mentions his birthday being 7 months before October 20, which would be sometime in March. In series 4, when Gunther asks Rachel when her birthday is, he starts to say "Mine's December ..." before Gunther cuts him off. In series 9, he reminds Joey that his birthday is October 18.
The birthdays and ages of the group are inconsistent throughout the show.
Given the salaries in the careers they each have, it is highly unlikely they can live as comfortably as they appear to since the cost of living in New York City is very expensive.
In many of the series episodes, when cast members leave Monica's apartment, you can see them turn left before the door closes behind them instead of right. Walking left out the door would be towards the audience and off stage and right would be out the sets apartment building.
Phoebe's birthday changes throughout the series. As early as the second season, we are told her birthday is on February 16 (Aquarius). However, during an eighth season episode, she and her friends are celebrating her birthday on October 31 (Scorpio).
None of the principal characters speaks with a New York accent.
On several episodes we are told that Gunther is Dutch. He even speaks some Dutch lines. However, Gunther is a typical German name, and in the Netherlands only people immigrated from German-speaking countries or with one or two German parents are named Gunther.
Lisa Kudrow visably flinches after doing one of the "Phoebe being mistaken for her pornstar sister" jokes.