Some ATM models have secret codes that can enter operator mode. A factory-preset code is sometimes published in the operator's manual of the ATM; this is changed to a personalized code by the owner.
Owners sometimes do not change the factory preset code of their ATMs, and criminals exploit this fact. The codes generally do not make the machine dispense money, but denomination settings can be changed so that the machine gives out a $20 bill in place of each $1 bill. It may also be possible to change the surcharge amount and view the total amount of money in the machine.