The oldest fiat currency still in circulation, the British Pound Sterling, dates from 1694. Since then, an additional 774 fiat currencies have been created. About one-quarter of them are still around, but most no longer exist. They failed due to hyperinflation, war, politics, monetary reform, or a combination of these factors. And the pound has lost about 99.5% of its original value relative to gold.
One example of a failed fiat currency you've probably heard about is the old German mark. In 1914, when World War I began, the mark was worth about $4.20. As we know, things didn't go well for Germany in World War I. By the time Germany capitulated in 1918, the mark had lost nearly 50% of its value. It fell from 4.2 to 7.9 marks per dollar.
But that was only the beginning. By the end of 1919, the mark-dollar exchange rate fell to 48 to 1. By the first half of 1921, it was 90 to 1. By the end of 1922, it was 7,400 to 1. In early 1922, you could buy a loaf of bread for 160 marks. But by the end of 1923, that same loaf cost 200 billion marks. Meanwhile, the USD to mark exchange rate fell to 4.21 trillion to 1.