Friends, a sitcom about six friends living in New York and overcoming the obstacles of life together, still remains incredibly popular today despite the fact that it ended back in 2004.
It's so popular in fact that talk of a possible Friends reunion special has got fans in a spin.
And as we eagerly await more news on the matter, we've kept ourselves entertained by re-watching the show on Netflix for the millionth time.
But on this occasion, during the re-watch, we noticed something rather odd about Rachel (Jennifer Aniston) in an episode from the show's ninth season.
Namely that for a few seconds in episode four, The One with the Sharks, Rachel is briefly replaced mid-scene by someone else.
Around 12 minutes and 20 seconds into the episode, Monica (Courteney Cox) and Rachel are in Monica's apartment, talking about unusual fetishes their previous partners have had.
Joey (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.
But 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.
It's a blink and you'll miss it moment though, as a few seconds later, the camera changes to focus on Rachel and once again, the stand-in no where to be seen.
This isn't the first time a stand-in mishap has been spotted on the show.
Monica was also replaced with a stand-in in season eight, episode five, The One with Rachel's Date.
Here Monica and Phoebe (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. How strange!
