A “miscut” is a defect caused by the trimming process, where the cover or pages are not cut perfectly square.

When a book is cut or “trimmed” it goes through two separate stages of cuts. The first cut trims the right edge (side opposite of the spine), and the second stage cuts the top and bottom edge (both are trimmed simultaneously). The miscut comes as a result of the first cut on the right edge. Whatever movement the book made due to being cut from the first blade is most likely the amount of miscut that is seen from the miscut of the top and bottom edges. This movement is what gives the misalignment for the final cuts.