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.