Considering how much work actually goes into making a movie, it's no surprise that plenty of mistakes or "happy accidents" happen on set. Most of those mistakes, though, will never see the light of day.
Some of them, on the other hand, actually do make it into the final cut, if only because they work extremely well for the film. When it comes to the ones in this list, you have to admit that the movies were better for the mistakes being left in.