A fan of the TV show "Friends" noticed that in Season 9, Episode 15, Jennifer Aniston's character Rachel was replaced by her understudy/stand-in in one scene.

This error went unnoticed for over a decade until it was pointed out in 2015. This isn't the first time an accidental swap happened in the show; in Season 9, Episode 4, a different woman is seen sitting next to Monica instead of Rachel. Fans never noticed this blunder either.