Librarians see this distressingly often. Every sheet of paper which is to become a "gathering" within a book should be printed on both sides before it is folded and cut. Sometimes the print fails on one side of the paper, resulting in what is called an "imperfect copy". Bookshops will readily exchange these imperfect copies for good copies, since the printer has to recompense them. If borrowers see this in a Library book, please tell the Librarians, as the imperfect copy is useless.
Sometimes a gathering is left out or duplicated, or both. Same applies