The cancelled leaf may be rejected on account of spelling or typing errors, a wording change or addition, or need to suppress libellous or otherwise offensive text.
Leaves have been cancelled from around the dawn of printing in the mid-15th century, peaked from about 1600 to 1800, but the practice is less common today, since it is now cheaper to replace the whole sheet.
If the replacement is made before publication, the book can exist in two states (i.e. with and without the new leaf).
If after sale, from a new leaf supplied by the binder to the purchaser, it is termed an alternative issue.
Cancels are not always easy to detect, if the stub or fold they are attached to is small; however, often there will be a difference from other leaves in matters such as paper thickness, watermarks, chain-lines, etc.