“For him that stealeth, or borroweth and returneth not, this book from its owner, let it change into a serpent in his hand & rend him. Let him be struck with palsy & all his members blasted. Let him languish in pain crying aloud for mercy, & let there be no surcease to his agony till he sing in dissolution. Let bookworms gnaw his entrails in token of the Worm that dieth not, & when at last he goeth to his final punishment, let the flames of Hell consume him for ever.”
- from the library of the monastery of San Pedro, Barcelona
It appears that some people have always taken libraries as a free book shop. I'd imagine that these threats of damnation did about as much good as our threats of fines to students. At one point a student was not allowed to graduate until they had paid all of their library fines, which became a motivating factor, however, one clever law student pointed out this was actually illegal and most institutions stopped enforcing this. Still, most libraries would be wise to not let this fact be well known.
- from the library of the monastery of San Pedro, Barcelona