The Coin Shaving Problem:
The right hand side quarter has had about 0.040" shaved off its diameter in order to trick the machine into paying out coins from the hopper without counting them. Most casinos have fallen victim to this scheme for emptying the slot's hopper without know it was happening. The change in coin diameter is small enough to escape casual visual detection. Simple coin comparators cannot distinguish such slight coin diameter changes and can be tricked into accepting shaved coins by "slamming" them. Shaved coins go in for credit and come out free. All IDX Coin Acceptors feature precision optical diameter measurement to prevent their acceptance in the first place!