I'm going to disagree with this, but purely based upon my experiences. I have minimal serious muay thai training and very rarely work my stand up consistently. I've sparred with a few pro boxers that have passed through a gym I was training at. Now I don't know a ton about boxing but I held my own, fought well and dropped one(the crowning moment of my striking career). I don't think it's impossible to land that one clean shot, granted I've been known to have a hard head and be able to take a lot of punishment combined with pretty heavy hands, not to mention I stand southpaw. But if I can drop a boxer in a pure boxing match I don't think it's impossible to think someone couldn't get lucky once.
I don't think it's likely, but anybody can be hit.
There is some truth to what you're saying. There really is always a chance and anyone can be hit. I could KO Jon jones. Probably not even if we fought 100 times, but there is a chance.
That "untrained guy" was Lee Murray, who went the distance with Anderson Silva. He wasnt some untrained chump off the street, who was actually very well trained. Also, there is no real way to know how true that story is. Good try though.
Lee Murray, legendary street fighter turned pro fighter. He fought anderson silva and he did alright, lost via UD. But to be fair he fought silva two years after the incident, he only had like 7-8 pro fights under his belt at that time.