- The USE CORRECT CHANGE light is wired to light when there is no money in the machine at all
- After you make your selection, the machine disables the count of money in the machine and the light goes on
- If you hit the refund lever while the light is on, nothing will happen because the machine thinks there is no money available to refund
- Your snack or drink comes out and the money count is enabled again
This is to prevent the machine issuing both a product and a refund; it's a race-condition avoidance mechanism.
However, this is just a theory, and surprisingly I can't find any information about it online. Anyone have any competing theories or authoritative info?