I think what caused Ned Flanders to snap was Homer Simpson borrowing his lawnmower and not giving it back.
Othwerwise he was always how diddly ooodling.
Thing is you dont know when someone borrows something whether they will bring it back. You just hope they will. They say they will but theres every chance that they may forget. But then it will be on their conscience not yours, otherwise, you cant be a librarian (or at least, dont work in circulation!)
You are better off being a bookseller.