Schwarzfahren

In Germany, you don't have to show any ticket or pass to get on a bus. You just hop on, and it's a bit of a honors system.
"A bit" because there are plain-clothed enforcers who can require you to show your ticket/pass. And getting caught without a ticket (called "schwartzfahren," or "black riding") gets you a 60-euro fine. This was not much of an issue during the summer, because a monthly pass was only 9 euros, and it applied to trains as well. However, now that the summer program is finished, I imagine that enforcement checks will have to go back up.