I'd heard about this, although I never knew the name of the guy who was responsible for it. It's the story of how a simple mistake led the now classic "It's A Wonderful Life," once a huge flop, to become one of the biggest movies of the Christmas season. And why there was a time it was on almost every TV station. 

How the Biggest Whoops in Cinematic History Revived ‘It’s a Wonderful Life’

And by the way, the cartoon below took me a second. But if you've seen the movie, you'll probably get it! Because every time a bell rings, well, you know...