I have fallen three times due to unclipping. Once in front of a gas station while I was sitting at a stop sign. I was trying to be that great rider that does not need to unclip and can stay stationary with both feet on the pedals. I guess I am not that rider.

My cleat came off my shoe once on a long ride. That was a fun day, my foot slipped off the pedal constantly. I tried riding a tandem once that had normal platform pedals. My shoe came untied and wrapped around the chain. I fell to the ground with amazing speed. The bike came shortly after, along with my riding partner (both on top of me) I was riding a bike once that did not have clips or clipless pedals. It was raining real hard and my foot came off the pedal and some how found it's way into the front spokes. Well I went head over handle bars and landed in the middle of the road. No one stopped to help, they just slowed down, pointed and laughed. I can't say I blame them. Ride on and have fun.