Properly designed slots which work with true Random Number Generators are theoretically unbeatable. But in reality, there are many documented cases of players who managed to trick slots into paying them out much more than the casino owners’ had hoped.
Many of these tricks are blatant theft. These usually include tools to manipulate a slot machine’s hardware, namely its coin dispenser or note acceptor. Some cases involved a casino insider who helped to manipulate the slot machine.
In another group of cases, players were just unusually lucky. They managed to find a software glitch and used it in their favor.
However, by far the most interesting case is the story from the very recent past (2009-2018) of a Russian guy from St. Petersburg, who managed to successfully predict the spin outcome on certain models of slot machines and used this to his advantage to milk millions from casinos around the world.