Correct, it may seem impossible not only to find those bugs but to the bugs themselves to exist at all, but as you mention the more familiar a person is with a particular platform the easier it will be for them to notice a glitch they may be able to use for their own benefit.
And a great example of this are video games, in which fans of a particular game are able to break it completely just days after it has been released, as they are experts on finding bugs they can use in order to end the video game the fastest or get the highest score.
Before the casino launches their project, they will release in a UAT or the user acceptance testing that will bring into their Quality Assurance team or the QA to test any bugs before launching to the production mode. During the live of course, we cannot deny that there's an instance that will show up with some bugs that didn't find in that time, in that case they will temporarily terminate the services of the game and fix those bugs to prevent getting abused by the players.
I have no doubts that casinos take this issue very seriously, but there are an infinite combinations of hardware and they cannot test them all, so their tests will always be incomplete and there is always the possibly a bug may appear out of nowhere where they least expected it, still if a gambler were to find such a bug they must refuse to give into the temptation as it is likely they will be discovered anyway and they will not be able to make a single withdrawal.
Exactly @Silberman that's why you'll see some sites shutdown temporarily and claim they're under maintenance, most times they've discovered bugs that seem treatful to them and have to temporarily close the app to fix them, no business man or woman would want to lose money hence why did they start up the business if not to make money, the casino is not a charity home, so they know there would always be treat and many would want to use different means, either by hacking into the sites or whatsoever, and since the casinos are aware about this they'll always maintain there security to fix bugs or hackers from penetrating.
Now back to the gamblers, refusing to give into temptation is something most people find difficult to do when it comes to making money quick, that's why about 50-60% of gamblers would want to take chances no matter the risk of getting caught, most of this people are poor or middle class people that's lost most of their money to gambling and would seize an opportunity if they discover any means of cheating the gambling platform to get rich quick or gey back their funds but the best way to avoid this temptation is by quiting even if it's not permanent, if you've lost enough money in gambling atleast pause for a while, study the game and how it works restrategise and you might be lucky when you try again after a while.