The One With the Rachel Who Wasn't Rachel.

An eagle-eyed fan of the hit show "Friends" spotted a mysterious non-cast member swapped in for Rachel Green (Jennifer Aniston) — something that went unnoticed for over a decade.

In Season 9, Episode 15, "The One With the Mugging," Aniston, 54, was seemingly replaced in a scene and fans had no idea.

The error was spotted by a fan who was rewatching the series starring Aniston as Rachel Green, David Schwimmer as Ross Geller, Courteney Cox as Monica Geller, Matt LeBlanc as Joey Tribbiani, Lisa Kudrow as Phoebe Buffay and Matthew Perry as Chandler Bing.

The discovery was first written about in 2015 on RecentlyHeard.

In the episode, Rachel runs into Monica's apartment to tell Joey that he got an audition with the fictional famous actor Leonard Hayes (Jeff Goldblum).

But when the camera pans back to Joey, someone who isn't Aniston is standing in Rachel's place.

This isn't the first time an accidental swap made it to the final cut.

In Season 9, Episode 4, "The One With the Sharks," Rachel was once again replaced in the middle of a scene.

At about 12 minutes and 20 seconds into the episode, Monica and Rachel are sitting at the table in Monica's apartment before Joey walks in and joins them, with Rachel sporting a yellow top and curly hair.

As the camera shifts over to Monica, on the bottom left side of the screen a different woman can be seen sitting next to her, wearing a blue top and straight hair.

The woman sitting at the table was clearly not rocking Rachel's look from just a few seconds prior, but, yet again, fans never noticed the blunder.