Came across this gem in Daniel Ellsberg's Doomsday Machine (p. 115-6 of my copy):
Partly at Rubel’s urging, Secretary of Defense McNamara later compelled the Minuteman developers, against great resistance, to install the equivalent of an electronic lock on the Minuteman, such that it couldn’t be fired without the receipt of a coded message from higher headquarters.
Decades later, long after McNamara’s retirement, Bruce Blair, a former Minuteman launch control officer, informed the former secretary that the Air Force had ensured that the codes in the launch control centers were all set continuously at 00000000.
According to Blair, McNamara responded, “I am shocked, absolutely shocked and outraged. Who the hell authorized that?”
“What he had just learned from me,” Blair continues, “was that the locks had been installed, but everyone knew the combination. The Strategic Air Command (SAC) in Omaha quietly decided to set the 'locks' to all zeros in order to circumvent this safeguard.
During the early to mid-1970s, during my stint as a Minuteman launch officer, they still had not been changed. Our launch checklist in fact instructed us, the firing crew, to double-check the locking panel in our underground launch bunker to ensure that no digits other than zero had been inadvertently dialed into the panel. SAC remained far less concerned about unauthorized launches than about the potential of these safeguards to interfere with the implementation of wartime launch orders. And so the 'secret unlock code' during the height of the nuclear crises of the Cold War remained constant at 00000000.”
The command & control chapter is full of horrifying cases like this.