The bus holding the warheads is programmed to manoeuvre exactly to release each warhead in the precise position and course/speed for a ballistic re-entry exactly on target.
Any of the paramaneters don't match the warhead isn't activated, the batteries not started and the warheads remain multiply safed and in the bus. And if we fire a live armed missile in anger, I doubt an inert warhead and bus falling to earth will be the top of the problem list.
Well, that's OK then!
(I did write it was "according to The Sunday Times"... Journalistic accuracy strikes again.)
