WATCH: When Jennifer Aniston gets replaced in Friends
The mistake has gone unnoticed for 12 years.
One eagle-eyed Friends fan has noticed something no one else has noticed in over a decade.
According to an article on recentlyheard.com Jordan D'Amico was having a Friends binge when he came across a error that no one else seems to have picked up on in the 12 years since the episode broadcast.
In the episode titled 'The One with the Mugging' (Season 9, Episode 15) there is a scene involving Rachel, Joey and Monica, just a few minutes in.
Rachel runs over to Monica's apartment to tell Joey that he has got an audition with a famous actor. This is when the 'switch' appears to happen - watch above.
One moment it is Rachel (Jennifer Aniston) standing next to Joey, the camera then cuts to Monica and then back to Joey who is now standing next to a stranger!
While only part of her body can be seen, she's definitely not the lovely Jennifer, in fact she's wearing a different coloured top.
Since Jordan D'Amico spotted this last week other Friends fans have been trawling the web and found some other episodes where stand ins have made it to the final edit of the show, including a scene with Phoebe and Monica in Central Perk, when a stand in appears instead of Monica in one angle.
This appears in 'The One With Rachel's Date' (Season 8, Episode 5), watch the scene below.