At a company I used to work for, we had the badge reader/controller for
the building in the security officer's office instead of the ops room.
We had a failure (the Inauguration Day storm in the Pacific Northwest)
that lasted longer than the UPS the security officer had put on his
desktop. You guessed it... the building went into failsafe mode; only
keys worked for access. None of the ops had keys and we were effectively
locked out of the building. Somehow we got a hold of the security guy's
home number and he came in and we put in an extension cord. Pretty
stupid, but not as cool as your story.
Related, but not to power, one Monday (holiday) the building went into
Day Mode, which was quite a bit less secure than Night Mode. Someone had
forgotten to enter the impending Holiday as a Night/Weekend mode day.
The main doors, loading dock, and all internal doors were wide open all
day until someone noticed none of them were locked.