Mary Minnow, at the Library Law Blog has recently posted about a service called Library Elf. This service plugs into your local library's computer system so you can see what books you have checked out, when they are due back and what is the status of any holds you have. In using the system, Mary has found that you can see other patrons' records. She isn't happy about that.
See: LibraryLaw Blog: "This card is viewed by other accounts" - an update on the Library Elf and your privacy and LibraryLaw Blog: Breaking Discovery - Library Elf blasts a giant hole through privacy - and why I terminated my account.