You used to be able to spin a 10p into the 50p slot on a video game in my local stations buffet back in the early 80s and it would register as a 50p Shame I never got it to work on a fruit machine.
This is how i became a master on Out Run, an excellent trick.. Another good one was the old style space invaders games without a 'flip - door' on the return coin slot. If you put your finger underneath you could feel 2-3 slots - get a tuppence and flick it upwards through one of the slots and you got credit as if 10p was being inserted..