We had a big Prague Metro survey today. When you entered the station you got a special ticket and you gave it back at your end station. You can imagine all the trains full with people holding their special ticket in front of them :)
Last time this kind of survey happened was when we were young boys. So after the school we traveled all stations like crazy getting and returning the tickets everywhere having fun that there will be some weird results after final counting. But I guess the people doing the "counting" didn't care at all :)