After all these years and so many re-reruns, mistakes and Easter eggs are still being spotted in Friends - and the latest is a humdinger.

During season 9 episode 15 ‘The One With The Mugging’, Jennifer Aniston was replaced with a stand-in while her character Rachel was still in shot, and the understudy (?!) wasn’t even wearing the same outfit.

The error was spotted by fan Jordan D’Amico (more than a decade after the show finished) during a binge-viewing.

Rachel...

Not Rachel!

“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,” he wrote on RecentlyHeard.

“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!”

It's apparently not the first time in the show this happened either:

This continuity error threatens to prove more irksome to fans than all the other Friends mysteries. We’ve reached out to the creators to find out what happened.

Update: The plot thickens - is there some kind of multiverse Rachel?!

Best celebrity cameos in Friends Show all 20 1 /20 Best celebrity cameos in Friends Best celebrity cameos in Friends Brad Pitt, series 8 Brad Pitt played Will Colbert, the boy Rachel used to bully at school (we know, how could she?). The pair were still married in real life at the time Best celebrity cameos in Friends Robin Williams, series 3 Robin Williams and Billy Crystal played best friends who crashed the iconic orange couch in Friends coffee shop Central Perk (awkward). Williams' character began stressing over his suspicions that his wife was cheating on him, while the bemused group of friends listened in Best celebrity cameos in Friends Freddie Prinze Jr, series 9 Freddie Prinze Jr played Rachel and Ross' male nanny Sandy, much to Ross' initial disapproval. He had a tendency to cry but put on flute and puppet shows for Emma Best celebrity cameos in Friends Charlie Sheen, series 2 Charlie Sheen played Phoebe's former Navy man lover Ryan in 'The One with the Chicken Pox'. Ross' son Ben had chicken pox and the couple, who had never had the virus, ended up falling ill. Needless to say, Ryan didn't show up again Best celebrity cameos in Friends Danny Devito, series 10 Danny Devito starred as 50-year-old stripper 'Officer Roy Goodbody' at Phoebe's bachelorette party. He broke down in tears after the friends were initially unimpressed, bu then Phoebe told him to give it his all... Best celebrity cameos in Friends Elle Macpherson, season 6 Model Elle Macpherson made an appearance as Janine Lecroix, Joey's roommate and eventual girlfriend. No, we weren't quite convinced by that one either... Best celebrity cameos in Friends Jennifer Saunders, series 4 Absolutely Fabulous' Jennifer Saunders starred as Ross' mother-in-law to be, Emily's mother Andrea. Let's just say it's a good job he said the wrong name at the wedding Best celebrity cameos in Friends Bruce Willis, series 6 Bruce Willis starred as the father of Ross' girlfriend Elizabeth. He did not like Ross. In fact, he really, really didn't like Ross. But he did like Rachel. So that was all a bit awkward Best celebrity cameos in Friends Brooke Shields, series 2 Brooke Shields made one of the early cameo appearances as Joey's crazy stalker Erica. She believed he was actually Dr Drake Ramoray, the character he played on Days of Our Lives. Then there was some hand licking... Best celebrity cameos in Friends Alec Baldwin, series 8 Alec Baldwin played Phoebe's new boyfriend Parker in 'The One in Massapequa'. He drove everyone mad with his incessant OTT compliments and Phoebe demanded he 'be much less happy' Best celebrity cameos in Friends Sean Penn, series 8 Sean Penn played the fiance of Phoebe's twin sister Ursula. But when Phoebe met her brother-in-law to be she found out that Ursula had lied to him about, well, pretty much everything Best celebrity cameos in Friends Denise Richards, series 7 Denise Richards appeared in 'The One With Ross And Monica's Cousin' as Cassie. The Gellers hadn't seen her in years and her utter gorgeousness caused Ross to overstep the mark. To be fair, we don't blame him Best celebrity cameos in Friends Julia Roberts, series 2 Don't believe that Julia Roberts would ever date Chandler? That's because her character Susie 'Underpants' Moss was out for revenge after Chandler pulled up her skirt at school. He ended up wearing nothing but her knickers in a restaurant toilet Best celebrity cameos in Friends Jennifer Coolidge, season 10 As Phoebe and Monica's old roommate Amanda, Jennifer Coolidge was a bit of a handful (ok, that's an understatement). 'Look at me!'; 'Smell my neck!' were just some of her remarks. And then she went and told Monica that Phoebe had once attempted to 'cut her out' Best celebrity cameos in Friends Winona Ryder, series seven Winona Ryder played Melissa, one of Rachel's old school friends. The pair had shared a kiss during a sorority party. Years later, Melissa declared her undying love. Rachel said: 'I'm just a good kisser' Best celebrity cameos in Friends Greg Kinnear, series 10 Greg Kinnear played the boyfriend of Ross' new flame Charlie. Dr Benjamin Hobart was a Nobel Peace Prize-winning paleontologist - who also happened to run the grant committee Ross had applied to. Dr Hobart wanted Ross to break up with Charlie; but then Charlie realised she still loved him and they got together anyway Best celebrity cameos in Friends Paul Rudd, series 9 and 10 Paul Rudd played Mike Hannigan, Phoebe's boyfriend and eventual husband. They met after Joey failed to set up a blind date for his friend and then randomly shouted out 'Mike' in Central Perk. Well, it's an alternative to Tinder... Best celebrity cameos in Friends Dakota Fanning, series 10 Dakota Fanning played the precocious daughter of a couple whose house Monica and Chandler were viewing. She befriended Joey, who wasn't too happy about his friends moving to the suburbs Best celebrity cameos in Friends Jeff Goldblum, series 9 Jeff Goldblum played director Leonard Hayes. He was seriously impressed by Joey's audition for his play, but didn't realise his agitated performance was fuelled by a desperate need to pee. Until Joey relieved himself when the pair hugged... Best celebrity cameos in Friends Christina Applegate, series 9 Christina Applegate played Rachel's younger sister Amy. Rachel and Ross got pretty mad when she mistook Emma for a boy and kept calling her Emmitt. She insulted everyone else to boot