Maginot Line

A wonderful example of how well-laid plans can be screwed up by something you thought was good luck. Impressive line of military fortifications on the Franco-German border built in the years prior to 1940, designed to force the Germans to repeat the Schlieffen Plan of 1914 attack France through Belgium, if at all. And, indeed, this is precisely what the Germans had planned to do, and the French had planned to stop. And all would have been well for the French, if a German idiot flying out to brief a forward commander hadn't essentially sent a copy of the invasion plans to French high command, forcing the Germans to come up with a new plan. They did. It involved the Ardennes, which was the place the French hadn't bothered to seriously defend. Ouch.