Never change money on the street from unknown persons!
Attention No.1: Dodgers often give foreign banknotes (Hungarian forrints) not real Czech banknotes. 500 Hungarian forrints is 2 Euro. 500 CZK is 20 Euro, so be carefull. It happens special, if you are going into exchange offices on the street.
Attention No.2: In official change office ask for course in order to change a money. Some of them have 2 course for Euro. First one if you change more then 5.000 (five thousand) Euro course is around 25 CZK/Euro and second one, if you change less then 5.000 Euro, course can be around 17 CZK/Euro!
This is not Czech speciality. It´s job of people from Eastern Europe, Ex-russia countries and others.
Pictures from: CNB – Czech national bank