What are some notable blunders in history that resulted in huge loss?

When the Spanish were raping the New World in the 1500s for gold, they dumped enormous quantities of platinum into the ocean because it was the wrong kind of shiny metal. Nobody in Europe had any clue how valuable the stuff was, only that it was often used to counterfeit gold. But since it wasn't gold, or even silver, everyone thought it was worthless. This was exasperated by the fact that nobody could melt the stuff until the 1800s. But mostly it was just not yellow enough for the idiots at the time.