The amount of time spent in preparation won't matter if the plan doesn't work.
The same goes for the number of deadlines that were met and how creative the planners were and whether a number of top teams were involved.
The plan has to work.
And a key part of devising a plan that will work is knowing just what can cause it not to work.
This is not a mystery. Consider the disasters you've studied and you'll usually find they have one thing in common: a major flaw which was known but was ignored or downplayed.
That can easily happen when people fall in love with a plan.
Good planners need the eyes of an adversary.