Some slots machines may have a poorly designed RNG. For example it may have too little entropy, and its state may be calculated from multiple rounds of watching the symbols it displays.

In one of Kevin Mitnick's books—can't recall which one, maybe The Art Of Deception?—he interviews anonymous hackers about their exploits, and one of the chapters is about a group of 4 people who did just that. They bought used slot machines, reverse-engineered the RNG in the firmware, found it was vulnerable. They exploited it to win a few million dollars over many months/years and many casinos. They spread their winnings over time, and they stopped when they felt it was too risky to get caught by the casinos.