Rare 20 cent coins: A special misminting brings in 825 times as much. Some 1 euro coins can also fetch thousands of euros - as can certain 50 cent coins.
Anyone who discovers an incorrect embossing can try their luck as a private seller on the Internet - and use the prices of other offers as a guide. If you still have old currencies at home that are no longer used as a payment, you may have a little treasure lying around. In general, old D-Mark coins and banknotes can be converted into euros.