Everyone loves a good blooper reel in TV shows and movies, but when there's a mistake left in the final cut it's even better.
There are so many cuts in movies/TV shows that it is understandable why it happens. Sometimes there could be hours or days between takes so props can get moved around. While we often notice these subtle changes, we get over them.
Seen here in Frasier and in The Big Bang Theory, for example.
By now most people have seen every episode of Friends. In fact, I would go as far to say we've probably seen each episode more than one time. It ran for years back to back on E4 and now it's showing on Comedy Central everyday so it's hard to avoid it.
But, I bet you have never noticed this tiny, yet fairly significant, mistake in the popular sitcom. In season 9, episode 15, 'The One With The Muggin', some random woman appears on screen in place of Rachel Green. How on earth they did they not spot it?
So here we see Joey and Rachel chatting at the start of the show:
The camera cuts to Monica and then back to Joey and 'Rachel' but it's NOT Jennifer Aniston on screen, it's a stand in. She isn't even wearing the same shirt!
But wait there's more. It also happened in the previous season during the episode 'The One with Rachel's Date'. We see Phoebe and Monica having a chat in their usual haunt, Central Perk, when Courteney Cox's stand in can clearly be seen.