I have once. It was a sparring accident when I was 16 years old. I wasn't sleeping but my vision was gone and I couldn't move. I was sparring with my trainer who was far superior than me in the art of fighting, and he was letting me get off bit. One move he was very well known for was a spinning back kick. He would turn and throw it high in a sweeping motion, usually as a counter attack. Anyway as I said he was letting me get off so I started to get a little comfortable and pressured him. I saw him ducking away and kept throwing punches. then I saw him setting me up for the kick but it was too late. I knew I could not block it and it hit me square on the chin. I could feel my teeth clank together and for a split second I thought to myself, I ate that shit like nothing. Next thing you know, my ass slammed to the floor and it felt like my legs were in the air lol. As I said I wasn't sleeping. I was aware, but this had never happened to me before. I said, "I think you knocked me out" "I can't see." I could hear everyone talking to me but my vision was blurred for at least a minute. I guess it was a very mild concussion, as I don't recall any headaches afterwards. Anyway I know most of you are undefeated physical specimen, whom which no one would dare challenge, but for the real ones who spar and/fight tell me about your experience.