As a librarian, I know the helpless feeling of having someone asking for a book by its title and author, and still not finding it anywhere. In by far the most cases the lender remembers the title slightly (or completely) wrong, and this is what we politely try to tell them. But of course, once in a while the catalogue itself is wrong or incomplete. It's not supposed to happen but, just like anything that's not supposed to happen, from time to time it does. And that makes us look very inept and somewhat stupid, especially after we've politely encouraged the lender to think just a little harder. In such cases, we correct the catalogue and hope for it to be the last time.
I am also a book collector on my spare time. I specialise in maps and atlases, and I do keep them all organised in a catalogue. Not that I need to, because I know exactly where each one is at any given time. (My private collection is not at all as gargantuan as the one I am custodian of at work.) Still, it's not so much about finding the books on the shelves, as it's about discovering more about them. Extracting the basic bibliographic data, when and where each book is from, is not always as easy as it sounds.