Well bugger me. It turns out that there had been a server crash on the day I had off the other week. Library Service policy on systems maintenance (if Kevin's not in) being to pretend nothing's happened, nobody thought to report the crash to the management system's support team so that they could check that everything was OK before working on it was resumed. And nobody thought to leave me a note saying what had happened so that I could promptly clean up the debris and minimise the damage.
I had hoped that the scrambling of the borrower database might have been a lesson learnt from last time but obviously not.
I had been wondering why half the indexing had suddenly gone up the spout and some of the invoices weren't reconciling against additions to stock.