Many, many years ago, when The Chicago Manual of Style was universally referred to as the Chicago Manual but was still actually named A Manual of Style (the official name change came with the 13th edition in 1982), I was arguing with a colleague over some fine point (which I can now not remember). The Chicago Manual was our ultimate authority. However, the person I was arguing with was also quoting the Chicago Manual. And it was the same edition. We checked the reprint code on the copyright page, and it turns out we had different printings. The University of Chicago press had either made an error, or what seems more likely, changed their minds on a particular point of style, and instead of waiting for a new edition, they made the change in a new printing of the existing edition. I know that even many of my colleagues don’t know how to read a book’s reprint code, and if you don’t know about reprints and reprint codes, you will be utterly baffled to find two copies of a book that say different things.