From Aunt Susan (a.k.a. “Conan the librarian”):
900 years ago monasteries were tough on people failing to return books, even to the point of excommunicating them and places curses on their heads. For example:
“For him that stealeth, or borrow and returneth not this book from its owner, let it change into a serpent in his hand and rend him. Let him be struck with palsy, and all his members blasted. Let him languish in pain, cry aloud for mercy, and let there be no surcease in his agony til he sing his dissolution. Let bookworms gnaw his entrails in token of the worm that dieth not, and when at last he goeth to his final punishment, let the flames of hell consume him forever.”
So maybe I’ll use this curse the next time that someone (who shall remain nameless) asks to check out his books on my card because his is maxxed out. And somehow all the books on his card get turned in on time and all the books on my card end up overdue?