After watching every episode around a thousand times, we like to think we're pretty avid Friends fans - but even we didn't spot this.
In a clip below, you can see the moment Monica is replaced with a body double in the season eight episode, 'The One With Rachel's Date'.
In this particular episode, Phoebe visits Monica at Allesandro's, where she meets Monica's sous chef, Tim.
They hit it off and Monica sets them up on a date. But, Monica ends up wanting to fire Tim because he's slow (and he also set her pastry chef on fire).
Phoebe meanwhile, persuades her to give him another chance - which can be seen in the clip above. In it, she says: "Oops, looks like when he got the pastry chef he got you a bit too" - referring to Monica's hair.
Monica, clearly offended, responds: "I paid to have this done!"
It's at this point that Monica's stand-in can be seen! We are shook.
But why did the show have a stand-in in the first place? Well, typically, stand-ins are used in filming to help with lighting, angles and how things look on camera when actors are unavailable or not necessarily needed.
And viewers over on Twitter were surprised they hadn't noticed Monica's double, with one writing: "Busted: conversation with Monica and Phoebe, but when the camera cuts back to phoebe, they have a stand in for Monica."
Meanwhile another said: "Watching FRIENDS for the trillionth time and I'm on SE8 EP5 'The One with Rachel's Date' and Phoebe and Monica are talking at Central Perk and... Hello Courtney's stand-in. Oops."
"When Phoebe chats to Monica and you realise that isn't Courtney Cox, but a stand in," added another.
We can't believe we never spotted this!
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