The 0 euro bill

Why would Germany have a 0 euro note? Because the euro, as a currency, is pretty boring.
By comparison, the U.S. dollar has historical figures, famous buildings, and weird quasi-occult symbols (like that pyramid on the back of the $1 bill).

Similarly, the Romanian leu features people from Romania's past, and the objects with which they are associated. For example, the 50 lei note has images of an edelweiss flower, early aviator Aurel Vlaicu, and the airplane he designed.
The official euros have none of that stuff. Because they are used by a bunch of different countries, national symbols are out. So also are historical buildings, which are invariably located in a particular county. So the only things shown on euro bank notes are maps of Europe, doorways that don't exist anywhere, and vague bridge-like arches over water.
These are not the kind of things collectors are interested in. That's a bummer, because governments *like* seigniorage; they profit from the difference between the face value of a currency and the costs of its distribution. Think back to the U.S. quarters that featured the different U.S. states, territories, and public parks -- by providing people stuff to collect, the U.S. government essentially made money by making interesting money.

This is why Germany has a 0 euro note. They wanted the benefits of seigniorage, but couldn't legally put their cool stuff on the shared currency. So they printed out nifty pieces of paper -- like this one featuring the Wiesbaden Kurhaus -- that they sell for 3€, and that you can't spend on anything. Perfect collector's item.