This is neither a test nor a joke. They did it. Figure it out yourself.
https://www.rosenheim24.de/bayern/kein-scherz-wertvoll-null-euro-schein-8345905.html
http://www.n-tv.de/ratgeber/Gibt-es-einen-Null-Euro-Schein-article19856300.html
http://www.coinbooks.org/esylum_v19n32a28.html