Edward O. Thorp developed, through maths, a strategy for blackjack that reversed the house's advantage and allowed the player to make money reliably. Prior to publishing the details of his system in Beat the Dealer he travelled to Las Vegas to try out the system for real - and make a bit of cash on the side. He made some profit but found that his system mysteriously didn't work as well as it should have done.
Thorp had naively assumed that casinos would play fair. In fact, his croupiers cheated him, dealing duff hands to limit his success. It was only later when he returned to Vegas with a two professional magicians that he could detect sharp practice from croupiers (and change table) that he made money in earnest.
I think if an expert with a secret breakthrough can get cheated at his own game then probably anyone can. Thorp had utterly cracked the logical part of casino blackjack: making optimal choices. The bit he'd missed was the finesse part: that croupiers were very good at what is now called "close-up magic", more than neutering the advantage of his systematic optimal play.