I've only fallen twice on clipless pedals in 8 years of using them.
1) a driver ran a red light and almost hit me...dropping the bike into a "controlled" fall was the only option.
2) One of the machine screws came out of the cleat as I was trying to unclip (thus making it impossible to unclip with that foot) and I was too slow to unclip with my other foot in time to avoid falling over.
Both of these incidents occurred over 5 years ago though. I now make sure to periodically check the screws in my cleats and have gotten much better at balancing on the pedals.