Counterfeit Bill Scam
This happens sometimes in restaurants or small shops where you don't have an eye on your money. You give the waiter 50 euros and he comes back and tells you that your bill is fake. So the end result is that he change your bill and you have pay twitch. To avoid this try to use small bills to pay things because they are not often counterfeit. 99% of the places are safe but is something to keep in mind.