"Permissive Action Links. ... So in 1962, Robert McNamara ordered every nuclear weapon locked with numerical codes. Effect: None. Irritated by the restriction,

"Permissive Action Links. ... So in 1962, Robert McNamara ordered every nuclear weapon locked with numerical codes. Effect: None. Irritated by the restriction, Strategic Air Command set all the codes to strings of zeros. The Defense Department didn't learn of the subterfuge until 1977." The most egregious default password in history.