This is a scam which mostly occurs in poorer countries, where the local buses and trams use a hole-punch system for passengers to validate their tickets. Especially if you are taking the bus or tram from the airport (or the main train station) to downtown, a lot of ticket controllers use these routes on purpose, to easily find victims.
As soon as they spot you, they mysteriously find problems with your tickets, even if you have enough of them and you stamped them properly. You want an example?
They will ask for your ticket, which correctly has a three-hole punched from the device, but while they show you a fake ID with one hand, they will replace your ticket with a four-hole one, with the other hand.
Sometimes the controllers are fake and try to scam tourists into giving them a bribe so they won't "call the police".
Oftentimes, legitimate controllers are doing the same thing, trying to make a little extra money. This scam used to happen a lot in Eastern European countries, before they considered changing the system. Some of them (Poland) have already - the punching-hole system being replaced with the stamping of the time and the date on the ticket - but not all of them!
However, in Warsaw (Poland) be very careful while riding Bus 175 from the airport to downtown, as the pick-pocketing rate is alarming.
How to avoid: before you get there, find out what kind of system they use. Once you arrive and are ready for the bus trip, ask around to find out what’s the procedure in validating your ride. They could “smell” you from far away and use every single trick to make you a victim. Since they are doing this several times a day, they know exactly how to manipulate the situation.
If you know you are right, keep an eye on your ticket at all times and don’t be afraid to make a scene and call their bluff. Also, take a closer look at their ID. One last thing, ask yourself: are you the first person to be asked for his/her ticket?