This afternoon I visited a branch library with my sons and checked out a large number of books for them, using the self-checkout. On leaving, we set off the alarm, because at least one book had not been correctly scanned. A librarian seized the pile of books, and proceeded to determine the offending book by binary search using the alarm (his algorithm may not have worked completely if more than one book was unscanned, so he performed a final check on the original pile, after scanning the one he found). His colleague was amazed and said that everyone else uses (in effect) sequential search of the receipt, checking it with the books in hand. I was amazed that someone this clever was working as a librarian, and that I had finally found a real-life application of an algorithm – I try to give many such examples when teaching algorithms courses, but they always have a slightly contrived feel to them.