That approach has its own problems. Working Model, a physics simulator from Knowledge Revolution (Macintosh, mid-1990s) used a copy protection scheme where it asked for the last word from some page in the manual. Unfortunately, after the copy protection was set up, but before the product shipped, someone made a small change to the manual that caused repagination. Page numbers in the first two thirds of the manual still worked, so the product seemed to be OK. Those in the last third didn't. Fortunately, you could just restart the program and try again, hoping for an early page number.