I think there is an element of luck in ALL racing. Where it comes in & starts to effect things is where things get cloudy.
Personally i think it was a bit of both, Rossi did screw up as he himself said, BUT where the luck comes in is how he was injured, there are no certainties when it comes to falling off a motorcycle...trust me ive fell off enough to get it down to a bloody art form!
Good case is when Lorenzo had his 2 crashes, they were meteoric, both of em but he only broke ankles & had mild concussion, i think some poeple forget the injuries he suffered as they were quite nasty, but they only ruled him out of the race weekend he was taking part in, this is the element of luck in my opinion as Jorge probably hit the concrete not run off as hard as if not harder than Rossi did. He did well to ride a week later really - tough kid!
His mistake was to give it to much throttle on a cold tyre due to backing off (Not on a flyer & in practise), i guess you could argue that it was just down to luck that the tyre decided enough was enough but i put it down to a schoolboy error...one that he paid a very high price for.
He isnt unfallable & he has made mistakes before, he made one then & im 110% sure he will make many more but its part of how these guys find the limits & the sport they take part in. Best case is Stoner - Full throttle, if i crash ive found the limit! Simples.
As for the Lorenzo element, my take on it that if he was on a flyer or in quali or somin simlar, then yes he made a mistake under pressure from a rival & the situation (Valencia 06), but i think as he wasnt on a flyer i think he just made a schoolboy error & paid the second to ultimate price - lost the title.