I'm ashamed to admit that I'm a library fine scofflaw. I did return the books -- ages ago, but at the time didn't have $80 to pay the fine and just let it slide. Now, 20 years later I still haven't paid that fine and my borrowing privileges are revoked. If I didn't have access to the university library, I know I would manage to scrape the $$ together and pay it, but right now I'm focusing more on paying off my medical bills.
With the university library, they send out a notice once a book is 7 days overdue and then another one two weeks later. After that they will assess a "Lost Book Charge" of $125/book once it gets to 40 days overdue, which is definitely an incentive to return any checked out books. Further borrowing privileges are blocked and if the book isn't returned or fine paid, they can take further action by either (a) holding up registration for courses if a student or (b) deducting it from one's paycheck if you are a university employee. So far it's kept me on "good behavior" wrt to my library interactions in recent years, that is. About 15 years ago, I had my paycheck held hostage because I had managed to compile lost book charges that totaled more than my 2 weeks earnings. Needless to say, I cleared that up the very same day.