I worked as a circulation supervisor at a university library for a couple of years. We used a lot of undergrads to man the desk, and generally they were pretty reliable and trustworthy, although there were exceptions, of course. I hated firing people, but it had to be done occasionally. The one I remember the most (Robert) was actually one of our best workers, which was a shame. The circ. desk computer had two logins -- the student login, and the admin login (which allowed a lot more capability, obviously). We generally kept the computer logged in to the admin login, because it was a bother to have to transfer back and forth between them all the time. Robert had apparently seen me forgive someone's fines at some point, and so knew the process to do it.
What he didn't know is that I would get reports every week on various circulation related processes, one of which is a list of all fines forgiven with the time and date. When I saw the report that said that over $100 in fines had been forgiven from his account during a shift that he had worked, and I knew that neither I nor the other supervisor forgave them, I didn't have any other choice but to let him go. It was awful firing him -- he cried.