On the Fact that (According to Murray Rothbard's Seminal Work, "America's Great Depression") the Federal Reserve Inflated the Country's Money Supply by 62% from 1921 to 1929 and Then Compounded that Mistake by Extending Cheap Credit to America's Industries

You don't get a lot of smoking-guns in economics but this is definitely one (the Fed overstimulating U.S. markets and thereby causing the stock-market bubble that finally burst in 1929). If only we could learn from such foolishness, huh?