In the original Die Hard, one of the key points of Hans Gruber's plan is that the police (and specifically the FBI) must eventually be notified, and allowed to proceed as if it were a terrorist hostage situation. Gruber knew that the vault's final locking mechanism -- the electromagnetic seal -- could not have its power cut locally, so he was counting on the FBI to work with the electric company and have the building's power shut down from outside.
But when things come to that point outside the building, there's some confusion and resistance. Special Agent Johnson and the supervisor/foreman from the electric company get into a heated debate over the impact of the shutdown. The supervisor insists the only way to cut the building's power is by shutting down the entire grid, affecting 10 square blocks, which would send that whole portion of the city into chaos. He doesn't want to do it, and certainly doesn't want to be the one who has to answer for it. Johnson of course doesn't want to hear it and doesn't care about the impact; i.e. if the grid's gotta be shut down then shut it down already. "You want authorization? How about the United States f***ing government??" [All this, of course, rolls right in with what Gruber expected and wanted all along to get the vault opened.]
But as they yell & cuss it out, there's a worker from the company who's trying to get their attention, and failing. He keeps trying to say something to the effect of, "I can do it from here! I can cut the power right here!" Once, and once only, his supervisor hears and insists he's wrong. Other than that, no one's listening to a word he says. The end of it is that Johnson gets right in the guy's face and tells him, "Lose the grid or you lose your job!" The guy scowls, bites his tongue and does as he's told. The grid is shut down, the EM-seal drops, the vault opens.
Now, did I read that whole scene correctly, when I've concluded that:
The guy was right, he could cut the building's power locally.
If he'd been listened to and had been allowed to cut the power locally instead of calling for the grid shutdown, then whatever separate circuit the EM-seal was on, would have stayed live. That is, the vault would have stayed locked and Gruber's plan would have been hopelessly wrecked.
If I'm right, then this would probably be one of the best face-palm ironies I've ever seen in any movie. The whole crazy business from there on, the shootouts, mass destruction, McClane's over-the-top heroics, all of it could have been rendered unnecessary. All thanks to one unnamed city electric worker who actually knew his stuff and pulled the right switch.