I have. Well, I didn't completely fall unconcious, smelling salts saved me from that.
It was sometime in November 1996, I was at Parris Island, the recruit boxing. I was getting my ass kicked. Took a solid blow to my head, and I started drifting off... then things went black as I faded off and then BOOOM! smelling salts and I was instantly wide awake.
My vision was fucked for the rest of the day and I threw up a couple times.