Friends fans will be baffled to learn that Rachel Green was swapped in one episode, with it remaining unnoticed for more than a decade.
During season nine episode 15 of the beloved sitcom, which is titled The One With The Mugging, Jennifer Aniston was replaced - unbeknownst to fans.
One eagle-eyed fan spotted the filming blunder while binge-watching the mid-90s sitcom, which is a series that follows the lives of six reckless adults living in Manhattan.
Ross Geller (David Schwimmer), Rachel Green (Jennifer Aniston), Monica Geller (Courtney Cox), Joey Tribbiani (Matt LeBlanc), Chandler Bing (Matthew Perry), and Phoebe Buffay (Lisa Kudrow) are six twenty-somethings living in New York City, with their adventures documented throughout the NBC series.
With the fan-base of the series ever-growing, fans are constantly picking up on new nuggets and Easter eggs scattered throughout the 236 episodes.
And the most shocking unnoticed thing that has now been spotted by a fan, sees character Rachel still in shot at the same time as a random person thought to be her understudy - who wasn't even wearing the same clothing.
Jordan D’Amico, a super fan of the series spotted the blunder more than a decade after the show finished, with him writing about his discovery on RecentlyHeard.
Writing for the outlet, Jordan noted: "Only a few minutes into the episode, an enthusiastic Rachel rushes into Monica’s apartment to tell Joey that he got an audition with the famous and fictional actor, Leonard Hayes, played by Jeff Goldblum.
"The three friends admit to admiring the actor for his cell phone commercials in which he starred opposite a monkey.
"Monica zealously posits that perhaps the monkey will be at the audition. Joey reacts by saying, “Don’t make me more nervous than I already am!” and goes to sit back down.
“It’s at this point that… BAM!"
And this isn't the first time either, with an episode from the the show's ninth season - The One with the Sharks - sees Rachel briefly replaced mid-scene by someone else.
Around 12 minutes and 20 seconds into the episode, Monica and Rachel are in Monica's apartment, talking about unusual fetishes their previous partners have had before Joey walks in.
They all sit at the table and Rachel can be clearly seen with curly hair and wearing a yellow top.
But as the camera moves to focus on Monica, a different woman can be seen sitting next to her in the bottom left side of the screen.
The woman can be seen with straight hair and a dark blue top - so definitely not what Rachel wore and looked like mere seconds before.
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