Possibly one of the biggest waste of money in history was the Maginot Line. After World War 1 the French were fearful of another war with Germany and so they built large fortifications on the border. The French spent over 3 billion French francs to construct. It was truly an impenetrable fortification. So what did the Germans do?
The Germans simply went around the fortified line. Instead of attacking France directly, the Germans went through Netherlands and Belgium. The quick attack caught the allies in surprise, and the Germans almost ended the war at Dunkirk, but the Germans made the mistake of not attacking them directly. Anyways, the whole invasion of France lasted only 6 weeks until they finally surrendered. Probably the biggest L in history.