I must say I can't quite get it now Sauron.
The French built the "infallible" Maginot-line during the 30's because of their well based earlier experiences of warware with Germans. They just didn't foresee the German panzer generals to apply a tactic of
blitzkrieg which made the whole line of bastions and bunkers obsolete. People gear up to a war they know. Burning the bridge into a city that is built on a lake for defencive reasons is the first thing to come to one's mind. And even if it's been years I have read the Hobbit the last time I don't think Smaug's attacks were that frequent that the Laketowners would have been so used to it's attacks that they would have known exactly what was to come.
So no extra-assumptions but just a depiction of how people react to a threat - even if that reaction is not the best one considering the opposition they face. Or should all characters in an epic story only behave in the optimal way? You can't possibly require that. And what would be the fun or excitement of a story where every actor was infallible and doing only the "right thing"?
