Rare 20 cent coins: special misminting brings 825 times the value. 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 want to sell a coin or want to buy a rare coin, you should be sufficiently sure that it is not a counterfeit and be informed about the private purchases.