Everyone who's much of a fighter (street) has been down at least once or they don't fight much.
I got my button punched once in grade school. I've seen stars a few times. I've put the lumps on a few. There's always the jack-off who sucker punches. I liked sending those a-holes to the ER.
I'm to old to be interested in mixing it up for fun anymore. No matter how big and tough you are, there's someone out there who's bigger and tougher.