This is actually a really interesting story that I saw on one of those "Vegas's Biggest Criminals" type shows. This Ron guy was given the job and created a device that would plug into the slot machine and check for tampering. He programmed it to check the device, mark it as clean, then insert his own code. This code allowed him to instantly win jackpots. He would have to bet a specific combination to open the jackpot. For example he had to bet 1 credit, bet 3 credits, bet 4 credits, bet 1 credit etc. I believe the sequence was like 50 steps long. He would obviously have friends play the machine to win.
Benefits to this system was that his team could spread out among many machines. If the casino wanted to check the machine, they would send Ron in and the machine would come up as clean (he developed the software to check it). The sequence was so long that it would take someone a lot of work to recognize it, and no one was likely going to stumble upon it by accident. It was really genius, however I can't remember how he got caught.