The safe in question contained a smartcard required to boot an HSM. The safe combination was stored in a secret manager that depended on that HSM.

The engineer attempted to restart the service, but did not know that a restart required a hardware security module (HSM) smart card. These smart cards were stored in multiple safes in different Google offices across the globe, but not in New York City, where the on-call engineer was located. When the service failed to restart, the engineer contacted a colleague in Australia to retrieve a smart card. To their great dismay, the engineer in Australia could not open the safe because the combination was stored in the now-offline password manager.

Source: Chapter 1 of "Building Secure and Reliable Systems" (https://sre.google/static/pdf/building_secure_and_reliable_s... size warning: 9 MB)