This Situation Happened to @AnaHilarski and I Last Year in Prague.
We had just arrived by train from Berlin. We were excited to be in Prague since we had never been there before and one of my friends lived there. So we had our guard down when an elderly gentleman walked up to me and asked if I need some Czech money. We said yes and I bought 100 Euro's worth.
We then hailed a cab for our trip to the AirBNB. When we arrived I went to pay the taxi driver and he told me that we had been scammed because the currency was Belarussian Rubles which are worthless! So we gave the cab driver some Euros. Later we tried to go to an official money changing center and were told that our bills were worthless. It really was a downer even though it was only 100 Euros.