I knocked someone the fuck out, twice, though it was not the same person.
Once when I was playing soccer I bounced my cleated foot off of anothers head playing the ball. He was unconscious for about 10 minutes and except for a large lump on his temple and a black eye he was quite fine.
The next guy I was in a fight with, big huge skinhead asshole, I knocked him out for about 10 minutes as well with a well placed fist to his noggin(TAKE A NOTE---HITTING SOMEONES HEAD WITH YOUR FIST HURTS LIKE A BITCH). He recovered as well with a hell of a headache
So in truth you can knock people out BUT I would never recommend it. It can cause serious injury.
Besides it is fucking hollywood, as long as it looks good and gives the writer an easy out why the hell not. I always suspend my belief at the doors of any movie theater.