There are numerous illegal ways in which slots have been cheated in the past. Most of the recorded slot machine frauds have something to do with hardware manipulation and/or special tools used to "fool" the slot machine and make money. In case you are playing slot machines with bitcoin on the internet, there are no ways to manipulate it. Only if you are the developer of that slot.
It’s important to point out that hardware manipulations and the usage of any tools to change the way slot machines work are illegal. That means that you might as well take an axe and smash the cabinet to get to the money storage area. From a legal standpoint, you would be doing pretty much the same thing.
Some of them occurred repeatedly, while others were a one-time operation. What all of them have in common is the fact that they only work with the physical slot machines you can find in land based casinos. Online casinos are protected, as players need to be physically near a slot machine to manipulate it.
Also, as casinos were being continuously defrauded, they started to implement further security measures and hardware updates that have made most of the frauds impossible to pull off today.