Alphacarina wrote:....which should have made the whole thing impossible....
Ah! Those classic words......
It is very interesting to look at the history of quality systems (or safety systems, or fool-proofing, or poka-yoke, or whatever you want to call it). We have some very early systems here in Britain from trying to make the railways safe, with things like the baton system, then the token system, then the token key system. Each step came from the result of an accident and prevented an error from causing another accident in the future.
To be fair to the guys handling nukes, they don't seem to have had many big mistakes even though they didn't get to learn from the same 'training' mistakes as other activities. OK, if you include the Russians, the score ain't perfect, but that's an important lesson: the Russians were historically very poor at feedback systems - they were good at top-down orders but piss-poor at bottom-up feedback on what works and what doesn't.
Er.... philosophy lesson over
Andrew