The game cost $2 to play. It loosely followed the payout rules of the TV show "who want's to be a millionaire". There were about 10-12 levels and every 2-3 wins you'd get to a checkpoint where you could cash out or continue playing for more money.
Each level was slightly harder in difficulty/time with 3 "prize/payout" levels, which changed based on how much the machine had paid out recently. You could generally win anything from $10-$40 per play.
It had about 8 different games you could play (one being sports trivia). One of the games you could choose was "spot the difference" where you have 2 images side by side and you needed to touch the screen in a set amount of time to find the 5 differences.
The method my friend worked out to "hack" the machine was to cross his eyes and merge the 2 images into one, similar to the stereogram "Magic Eye" images you can find. By crossing his eyes and merging the images into a single one - the differences stood out really clearly.
This lasted a few months with him and his friends cashing in on all the machines in the area, whenever their prize level was high. Eventually the software company worked out there was a problem with that specific game and updated their software.
It didn't pay out as high on that game. And the top prize level had to be completed in under 2 seconds. Which when you're trying to press a not-so-great touch-screen 5 times in 2 seconds, was not possible as not all presses would register if you touched it really quickly.