This actually happened to me. I had just gotten up, I went downstairs in the living room and I slipped on some water on the floor - I'm guessing my dogs must've spilt some the night before. Anyway, I fell flat on my back and the impact was so strong, I don't think I'd ever felt anything like that before - I literally could not breathe for like 10 seconds. I was able to get back up not long after..