I fell once after going clipless and had a few other close calls. For me, it was never an issue of it not being easy to get out of the pedals, but rather forgetting about them. After a few years moving my heel to the side is just a natural reflex, but I remember the fall like it was yesterday. I stopped and tried to just lift my foot off the pedal, I had just done about 30 miles and basically forgot about what I needed to do. This was after about a week of riding clipless. I guess it was just long enough for the pedals to not be on the top of my mind, but not long enough for it to be natural.