How Did This Friends Slip-Up Go Unnoticed for 12 Years?

We've only watched every episode of Friends ever innumerable times, so how in the name of jaysis this continuity slip up passed us by unnoticed we'll never know. Thanks to an eagle-eyed internet user, here we have photographic evidence of 'The One Where Jennifer Aniston Turns Into a Completely Different Person', AKA 'The One With The Mugging' - from Season 9, episode 15. You may know it better as 'The One With Jeff Goldblum.'

Here, during the opening titles, you can see Jennifer Aniston's Rachel clearly standing beside Joey Tribbiani in Monica's apartment...

And then, when they cut back to Matt LeBlanc, who the hell is that?!

OK, so it's clearly a stand in, but the more pressing questions here include a) how did this make it into the cut, and b) how come the internet is only beginning to notice now - twelve years later?

Kudos to the people at RecentlyHeard.com for spotting this one.