Refund mechanics are the good part, and the sensor at the bottom is usually not a scale. On the old machines it is a mechanical flap in the chute that the falling bag slaps on its way past, and the flap closing the switch is the vend confirmation. Newer ones put an infrared beam across the same spot for the identical job. A scale would be overkill anyway: the machine does not care how much dropped, only that something did. Your door idea is real, but backwards from how you framed it. The delivery door is spring loaded and the controller watches it shut. A bag wedged so the door cannot close leaves that switch open, the machine logs a failed vend, and the coil gets marked sold out. That is exactly why the rhino works: you are not beating the detector, you are finishing a vend the machine had already written off. The part fiat cannot do is the refund itself. When a vend fails, the machine knows it took your money and handed over nothing, and the only way back is metal in a cup, so plenty of operators just keep the coin and eat the complaint. Run the same jam on a Lightning machine and nothing in the protocol stops it from sending the sats straight back to the payer. Same sensors, same stuck coil, but the operator finally has a refund that does not cost him a coin float. And I know which machine you are remembering. Some of those sold things that needed no shaking.