Back in the 90s, I wrote and maintain programs for Siemens and Diebold ATMs that interface them to the bank's mainframe system.
All ATM transactions are written to a database, and then at night, mainframe batch programs are run to reconcile these transactions with the customers accounts.
If transactions are heavy (example: weekends, payday) the ATM database gets full quicker, so the ATM can't write anymore transactions, and displays an Out of Service error message. All ATMs used by the bank will display this same error message. So basically, their ATM network will be dead.
Usually, there's a phone nearby the ATM where customers can call the bank's IT department to report any ATM related problems (card captured, ATM dispensed wrong amount, bin out of bills, etc).
At the mall, I was trying to withdraw money that day, and there was a long line of frustrated users that can't withdraw their money. The bank having that problem is one of our company's customers, so I picked up the phone, told their IT department their ATM Journal is full and they need to execute such-and-such program, then switch to the secondary Journal database by executing such-and-such command on their terminals. The guy said "oops, you're right! Will do that now".... a few moments of silence then he said .... "HEY! How do you know all these confidential stuff?!" .... then I hung up the phone.