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

King Pyrrhus of Epirus. He was known for winning battles against superior armies, at the cost of taking heavy losses. He was once quoted as saying "If we are victorious in one more battle with the Romans, we shall be utterly ruined."

He was so famous for this, that the term for a victory that devastates the victor bears his name, a Pyrrhic victory.