The one where Monica was replaced on Friends and no one noticed

So, here's the thing. TV shows take a really long time to film – like, weeks and weeks. And sometimes there are scenes featuring a main character out of shot – so rather than dragging a famous actor out of their trailer to stand on set for hours on end, show bosses use a stand-in, usually someone who bears at least a passing resemblance to the character in question.

But occasionally – just occasionally – the stand-in slips into shot and eagle-eyed fans gleefully take to Twitter to point out the mistake. What's surprising is how long it can take us to spot the error, especially when it crops up in programmes we've watched over and over again – like Friends.