"A theorem: In matters of military contingency, the expected, precisely because it is expected, is not to be expected. Rationale: What we expect, we plan and provide for; what we plan and provide for, we thereby deter; what we deter does not happen. What does happen is what we did not deter, because we did not plan and provide for it, because we did not expect it."

Sir Michael Quinlan, MoD PUS 1998-2002, cited in 'The Secret State: preparing for the worst; 1945-2010' by Peter Hennessy. Thanks to Sam Leith, The Spectator.