i serviced coinstar and currency machines for years. the sensors are very sensitive will reject any foreign, non-monetary, or even extremely dirty/corroded coins. On most type of coin counting/sorting machines, there is a vibrating, pre-sorting area to clear out foreign objects, etc, fropm there, all the coins go into the sorting area, where they are passed, one at a time, in between a sensor array where they are measured on 5 different stats, size, density, and magnetic response basically. if a coin fails one of the those checks, it is kicked into the reject bin before it ever completely leaves the sensor area. depending on machine configuration, they are then either bagged by denomination, or dumped into a large single trolley vault. I would occasionally find canadian coins and game tokens in the shaker assembly, but it is pretty difficult for the coins to really make it much further in the the machine if there is an issue with it. while they are very complex machines and work very well at high speeds, they are only expected to be 70 or so percent efficient, so you do generally suffer appreciable loss on large transactions. the coin counters in bank lobbies are even worse, because they are serviced/maintained by the tellers, who don't give a damn.