I went down once or twice when I first started riding clipless.
Recently I went down technically because I could not get my foot out in time, but really because I was crossing a 100 yard stretch of road that is sand... not gravel, just sand. The problem Is that this stretch of road is between my house and the rest of the world... I will have to post pics one of these days.

Nothing like thinking you can make it across because the sand is damp and packed down , then finding your front wheel hit a dry spot and dive about 3 inches deep.