This scam takes place in most of the European capitals’ subway stations, but it could easily occur in any other country, so beware.
Whenever you are purchasing tickets from the dispensers located in the underground, somebody will come and help you, while you are trying to figure out the machine.
Besides the employers of the transit system and occasional genuine helpers, scammers will approach tourists, willing to “help”. While they are pressing the buttons and asking you where you’re from, they will also be quick in switching your multi-day pass just printed with the one-way pass, already in their hand. Minutes later, they will sell your original ticket to somebody else.
How to avoid: if you really can’t figure out the dispenser, ask the real employees of the subway system, usually dressed in uniforms. If you don’t see any and someone else helps you, don't take your eyes off the ticket from the second it is printed out.