The only problem I've ever had on clipless was when I lost a cleat bolt on my SPDs which caused the cleat to stick in the pedal and not rotate with my shoe so that I couldn't unclip. I ended up on the ground, which wasn't a big deal because I frequently end up on the ground anyways because I'm a hack. That's the only time that my clipless pedals have caused me to fall or contributed in some way.

Before I got 'em however I can't count the number of times my feet slipped off the pedals causing me to take a seat nose or top bar to the pills...