It has been on our TV screens since the mid 1990s with many of us knowing the words to our favourite scenes off by heart. But despite watching Jennifer Aniston, Courteney Cox, Lisa Kudrow, Matt LeBlanc, Matthew Perry and David Schwimmer more times than we could care to admit, some fans are still spotting mistakes and mix-ups they've never noticed before.
With David Schwimmer, AKA Ross, baking up a storm on The Great British Bake Off many people have found themselves searching out the old episodes - and those tuning into series nine have spotted something very, very odd about everyone's favourite fashion expert - Rachel Green.
For a few seconds in episode four, The One with the Sharks, Rachel is briefly replaced mid-scene by someone else.
About 12 minutes and 20 seconds into the episode, Monica, played by Courteney Cox, and Rachel are in Monica's apartment, talking about unusual fetishes their previous partners have had.
Joey, portrayed by Matt LeBlanc, then enters and heads towards them, explaining that he thinks he has slept with his date before - but she doesn't remember him.
The three sit down at the kitchen table to continue the conversation and at this point, Rachel can be clearly seen with curly hair and wearing a yellow top.
As the camera moves to focus on Monica, eagle-eyed viewers might notice that a different woman is next to her in the bottom left side of the screen, with straight hair and a dark blue top.
But if you blink, you'll miss it as a few seconds later, the camera changes to focus on Rachel and the stand-in is no where to be seen.
This isn't the first time a stand-in mishap has been spotted on the show, as Monica was also replaced with a stand-in in season eight, episode five, The One with Rachel's Date.
During this scene, Monica and Phoebe, played by Lisa Kudrow, were having a chat on the orange sofa in Central Perk and the camera switches between them as they talk.
Towards the end of the conversation, when the camera focuses on Phoebe, it shows she is sat with a totally random woman, who looks nothing like actress Courteney Cox.
Similarly, Rachel was replaced in yet another episode of season nine, The One with the Mugging. In this one Rachel is seen running into Monica and Chandler's apartment to tell Joey that he got a big audition.
She stands in the door way a little behind Joey, but when the camera comes closer to him, someone else wearing a similar outfit to Rachel, can be seen standing near the door.
David Schwimmer appeared on Channel 4's The Great British Bake Off on Sunday, March 19.
Have you spotted a movie mistake? Let us know in the comments below.
