Special 20 cent coins: Rare misminting brings in multiples of value. Some 1 euro coins can also fetch thousands of euros - as can certain 50 cent coins.
It's not just euro and D-Marks that are worth a lot: rare coins that are D-Marked can also be worthwhile. If you still have old currencies at home that are no longer used today, you may have a little treasure lying around. In general, old coins and notes can be converted into euros - but it may be worth exchanging them instead.