By now, you should have finished your epic binge of Friends on Netflix. It has been on the streaming site for almost a year, you know, which is more than enough time to get through all 236 half-hour episodes. Congrats if you’ve finished, and keep chugging along if you’re still watching. But right now you have to stop what you’re doing because there’s one episode you have to go back and watch immediately. It’s Season 9, Episode 15, “The One With The Mugging.” That’s because in this episode, Jennifer Aniston doesn’t play Rachel Green.

I swear on all the Friends episodes where Paul Rudd plays Mike, this is true. This is the episode where Ross realizes that it was Phoebe who mugged him in Central Park all those years ago, and, elsewhere, Jeff Goldblum guest stars as a director working with Joey. And also, as one eagle-eyed viewer has pointed out, for a split second of time Rachel Green is a completely different woman.

Two minutes into this episode, Joey comes rushing into Monica’s apartment with Rachel right behind him. They have a quick back and forth, and then Joey goes to sit down at the table with Monica. Rachel is barely in frame at this point, and this is when she goes through a crazy split second transformation and becomes a shorter woman with brown hair.

Don’t believe it? Here is our friend Rachel, as played by Aniston.

Moments later, here is our friend Rachel, NOT PLAYED BY ANISTON.

Who is this woman, and what has she done with Rachel? Why isn’t she at least wearing the same shirt as Rachel? What’s going on? The answer: She’s most likely a stand-in. Maybe she jumped in for a quick second when Aniston had to step out of the scene for some reason. Maybe this scene needed to be re-shot and Aniston was unavailable. There are lots of reasons a production might choose to use a stand-in while filming a scene. Whatever the reason, there is no denying that she’s not Jennifer Aniston, and clearly accidentally made it halfway into a Friends episode.

So there you have it. This is now “The One Where We Totally Missed A Crazy Tiny Detail.”