There is no such thing as a lucky punch, since the fighter threw the punch with the intention of it landing on the opponent.
If, however, the fighter didn't throw a punch at all and the opponent still got knocked out... That is a lucky punch. :P
What about the punches the fighter doesn't intend?
Training can lead to automation, when a fighter is rocked, he might throw an unconscious punch and hit. Again, if he connects, one would have to say it was rather lucky, but one would also have to credit the training involved to cement that automation.
In this case, intent at the moment, has nothing to do whatsoever with the 'luck' of the outcome.