This string has been haunting me for a while, it never changes, but asks for translation after every update of the po files: Finally, if you charge a rental fee for the item type and want to give a specific patron category a discount on that fee, enter the percentage discount (without the %% symbol) in the 'Rental Discount' field Maybe a problem with the percentage sign?
*** Bug 19132 has been marked as a duplicate of this bug. ***
Haven't seen the string in question in a while - appears to have been fixed!