With the old 3-reel mechanical slots, I think the cheats found a way to freeze the first 2 reels on the Jackpot symbol, then insert coin(s) and spin the 3rd reel until it reads Jackpot too. Quickly remove the tool that froze the other reels and you have a Jackpot payout with only a few coins invested. (Or something like that.)

I don't know precisely how it was done, but seem to recall a discussion about "gaffing" such machines. Pre-camera era.