When I was younger people used to use lawnmover strimmer cable to clock up credits on arcade cabinets/fruit machines instead of using money. We used to have a hook in the cable and shove it down the coin slot and pull it up and down to catch the coin mechanism and fool it to thinking money had passed though. Another way we dd this was to use a voltage generator such as one of those things that makes a spark to light a gas cooker, we could send a pulse down a metal chasis to do the same thing.
On old satellite TV boxes you could use a sensor to pick up magentic fields on IC's to see what data was being sent - this trick has been used to capture data and then code CAM emulators to bypass smart card secutriy on quite a few set top boxes.
On older mobile phones, the phone credit was stored on an eeprom, you could wire in your own chip with a set value and turn off the write feature so you could have infinate credit.
Basically there's ways to bypass everything if you have an understanding of electronics and software, from using paperclips to short things, electrical pulses to short things, magnets and sensors to capture data, mechanical devices to pull switches, code injection to re-write values or bypass checks.....the list is endless.
