The National Bank of Poland (Narodowy Bank Polski) have issued a limited edition (35,000 issued) or ‘collectors edition’ bank note to commemorate the 1050th Anniversary of the Baptism of Poland with a face value of 20 złoty.
If you are lucky enough to get one in your change, don’t worry, it’s legal tender, but you would be well advised to keep hold of it.
As the Banks website states:-
“Collector banknotes can be used on the same principles as the general circulation banknotes, i.e. they are legal tender in Poland. It is worth noting that numismatic items (collector coins and banknotes) are valued much higher in the collectors’ market than their face value. The holder of a collector banknote should be aware of the fact that paying with such a banknote may lead to a significant financial loss.”
If you are a collector you can read more information in a brochure detailing the new note. (English)