Having ridden clipless on my MTB's for years I rented a bike at Laggan Wolftrax with flats and had a spin round the red route a couple of weeks ago. I couldnt believe how often my feet came of the pedals on the jumps and rocky sections. My scraped shins are proof that there is a technique to keeping feet on pedals that I havent mastered. Like others have said once you learn to clip out you do it instintviley when coming off. I have always found its more of an issue on steep or technical uphills when you need to get your foot down but have still got too much pressure on the pedals.