It's almost impossible to flip a bike over without doing it on purpose. As someone else said, almost all cases are actually caused by people not pushing back with their arms and flying over the handlebars, rather than the bike itself flipping over. Practice using both and you will notice that the rear brake is almost useless compared to the front. It may seem adequate if you've gone your entire life afraid to touch the front brake, but it will always be much weaker. It's simply too easy to skid the rear tire because of the weight transfer that occurs during stopping.