How real is the SPD accidental release issue?

I recently started riding fixed with front brake. Shimano SPD cleats and M520 pedals. First day I used very worn (thousands of miles, thousands of unclips) cleats and the left (my unclip foot) did pop out quite easily with hard braking. I put on new cleats and no matter how hard I tried I couldn't get them to pop out with a very hard pull. But will I find that even a slight bit of wear will make them prone to accidental pop out.

Al