My local library fines its patrons ten cents per overdue book per day. They will let you continue to borrow books as long as your fine balance is less than $10. As an academic, I sort of think of library fines as a cost of doing business, and I frequently carry a balance in library fines of a few dollars. (To be clear: I mostly incur these fines on books taken out for pleasure reading. What I mean is that I don’t have a moralistic attitude about my library fines.) Since there are no interest charges or time limits, I can carry such a balance for months on end. I’m following the rules, not cheating anyone, and I have never felt bad about my habit of being a few bucks in debt to the library.