I've been in a good many fights outside of the dojo. Lost three. Sustained closed head injuries in all three. All three were from behind. In one, an onlooker just couldn't stand it any more, jumped out of the crowd, coldcocked me, and then attacked the guy I was fighting
Amnesia for three days.
In another, I was suddenly hit from behind with a baseball bat...didn't know I was in a fight until then. Paralyzed my leg for a few minutes.
I don't remember what happened the third time for some reason.
Working on situatiional awareness should be your first priority.
I work as an Emergency Department RN. It's been several years since I've studied this question but I've seen quite a few posts here on the mechanics of a knockout that neuroscientists disproved quite some time ago. The last time I checked neuroscience was not absolutely sure of the mechanism(s) involved.
One thing to remember about such injury is that the damage is cumulative with each subsequent insult.
Now, don't get me wrong with my saying this for I think highly of Brazilian Jiujitsu, but I had a guy say to me once,"Brazilian Jiujitsu is the best one on one unarmed martial art in the world."
I replied,"That may well be true but it still leaves you with two problems-finding a way to fight me one on one and finding a way to fight me unarmed."