I've had the mechanical combos on ATM safes 'slip' far enough a few times to cause it not to open. Took trial and error to figure out which number 'slipped', in which direction and how far it slipped to get open. Basically a setscrew is all that keeps the combo numbers on commercial mechanical combo locks secure. The little slash mark off center on front of number dial plate is for resetting / changing combination. Takes a simple but special tool to change/reset combo.
When needed we also used a locksmith who specialized in drilling safes with tripped relockers, unknown or corrupted combo numbers.
It was a rather lengthy and involved process. He took exact measurements of locking mechanism and any other critical dimensions inside door of another identical model safe before drilling. The drilling itself took a good while. A lighted fiber-optic video camera was inserted through drilled hole to view locking mechanisms inner workings and the search for correct combo numbers began. When finished hole was welded up, painted and safe was ready to put in service again.
