Unexpected Moments That Changed The World

I'd like this to be a thread where people add their own examples.

In 1973/1974, Bruce Springsteen and the E Street Band have spent months rehearsing and recording the album Born To Run. But they're stuck on the title song. This whole thing should take a week, max. But Bruce just isn't happy. This is like 6 months in by now.

So, Steve Van Zant is the friend of a friend, who says 'you should check out my band'. Steve comes to a rehearsal. He may or may not even be aware of the problems. Anyways, he's there about 5 minutes and says, 'Have you guys tried doing it this way'? The rest is history.