Yep, twice if you count medical anesthesia. Had an operation once that put me out, but actual knock out I've had once too from ice hockey.
No, it wasn't a body check or anything like that, it was just an eleven year old who could skate very quick and yet not stop half as fast. Gym class, had a race see who was fastest as a warm up. Well, round the rink once then slam into the boards near the bench when I tried to stop. Regaining consciousness and looking up I found I really didn't slow down at all, but hey I won so it lessens the embarrassment.