I would offer that this is not true. While military training exercises try to replicate what will be done and what will happen when the 'real thing' comes, it can not and does not contain those unexpected events that occur in real warfare. One only needs to look at the Tarawa landings or even D-day examples.
As for your contention that Dieppe was intentioned to fail for political reasons, I would offer in reply that the entire operation was an on again, off again, and on again affair in which operation security was thrown to the winds. One can not plan an operation of that scale, brief the decision makers and commanders, and then leave it to be executed virtually unchanged at a later date without 'word getting around'.