The Single Switch That Saved the East Coast from Nuclear Disaster

The amount of known nuclear close calls is scary. The fact that disaster has almost struck multiple times shows how we shouldn't have nuclear weapons.

https://en.wikipedia.org/wiki/List_of_nuclear_close_calls

Surely the spread north would not have stopped at the border, as well.

so light on the safeties, what failed, why and what didn't and how?

I also assume the parachute on the bombs were not for safety.