Well, now, that's most certainly an incentive for turning your checked out books in when they're due. I'd say Tom found out that it's always the little things that trip you up. No doubt Madame Pince is going to get lots more respect from now on; and I suspect some long overdue books are suddenly going to appear from all sorts of families where they "just happened to be left in the trunk when I left"; be amsuing of the Malfoy library suddenly relocated to the Restricted Section of Hogwarts Library.