I recall a librarian trying to screw me over on overdue and replacement fees for a book when I was in my final year at university (they were, in not so many words, accusing me of stealing the book). They were employing the "you can't graduate with overdue fees" gambit. I
knew I'd returned the book (because I was borrowing 6 books at a time, and photocopying what I needed, and returning them within the hour; repeat about 10 times), so a series of emails were to-and-froed, until the librarian in question thought that I was becoming hostile and rude (it's not like they were threatening to stop me from completing a degree or anything

), and insisted on a face-to-face interview (with her supervisor).
I arrived early, and decided to do some investigating of my own, and to see if I could produce some evidence to prove my innocence (and to wipe any smug bureaucratic looks off their faces, and get an apology - I figure for people like that, rubbing their noses in it might teach them a lesson). I knew the copy that I had borrowed was the only copy the library had, so I went in to the stacks, armed with the book number and supreme arrogance (and aided by some high caffeine intake), to see if I could find the thing. Of course, it wasn't where it should have been among the BD books. So, I took the logical step of looking in the DB books (figuring that it would be simple enough to transpose the two numbers), expecting it not to be there, and then having to go through the A-Z, but it was. Took me all of 5 minutes to enter the library, and find the book on the stacks that had been 2 weeks worth of email-fighting-and-graduation threatening.
So, I did what any inquisitive student would do as they next step. I went to see if I could
borrow the book. I expected all sorts of bells and whistles to go off, or at least a confused library assistant, but, nope, it went like a normal transaction. I put the book in my bag (with the loan slip) and went to the meet with the librarian.
After brief introductions, and the librarian I had been dealing with giving me filthy looks as her superior did all the talking, I got a bit of a lecture from the supervisor about courtesy, and procedure and resolving issues, etc., which I sat and endured, smiling all the time (which made the librarian's body language even filthier), until it was my turn to speak. I explained my borrowing habits for the day, and made it clear that any fool could see that I was borrowing and returning books because I was using the staff room photocopier that I didn't have to pay for, rather than the library one, and that it seems odd that I would have kept one book, and continued to borrow and return others.
"Did anyone think to actually look for the book on the stacks, or did they just take the computer's word that the book was never returned?" I finally asked.
Of course, the librarian I had been dealing with retorted that they had, and that she resented being asked that in the emails, and now, and that she knew how to do her job, and...
... I cut her short by slamming the book on the table and saying:
"Because I found this in your stacks half an hour ago and borrowed it," presenting the loan slip. "Filed under DB, not DB. Took me 5 minutes to find."
She went scarlet - I honestly thought she was going to explode. Her supervisor looked like they wanted to facepalm. Sputtered excuses and explanations ensued, and half-apologies.
The book, by the way, was
Anarchy, State, and Utopia.