What you don’t realize is that page with the one error originally had a variety of errors: grammar, punctuation, word choice, verb tense, capitalization, cliché usage, awkward construction, to name a few possibilities. The red marks and lines that appeared on the page (as changes were tracked) effectively camouflaged that error. Or the editor may have been tired or sick, unintentionally doing less than stellar work. Or the error was “fixed,” but before the editor got around to saving the document, the computer shut itself off; a segment of corrections the editor had made were lost without the editor realizing it. Or the author made changes after a round of edits, and the editor’s quick review missed the new mistake. Or the editor fell into a pattern of seeking out a specific type of error and missed errors that fell outside the parameters. Or it’s not an error after all, and you are mistaken. (It’s true; I’ve had folks point out “mistakes” that were not mistakes according to the Chicago Manual of Style.)
Well, that makes it sound like the book must have been poorly written. But that is often not the case. If you write 97,500 words of a 350-page book, there are bound to be errors. I certainly don’t expect perfection. Even an editor, with editing software and other resources, cannot successfully write an error-free book. As a writer, you are focused on sharing a story or thought and on getting your message across. Your eyes skim the pages, often seeing what you meant to type rather than what you actually typed. Have you ever typed a Facebook status, read it and thought it was perfect, and then read it again the next day, only to see a mistake that you overlooked the day before? It’s like that, but on a larger scale, when you write a book. Furthermore, most people haven't memorized the forty-seven comma rules and other grammar rules that editors find so fascinating. Just as each writer has strengths, each writer has weaknesses as well. They are human, after all.
In the old days, there would have been an editorial team, multiple eyes scanning the pages for errors to correct. Today, editors are considered expendable, and it may be that only one is assigned to each project. Everyone is trying to cut costs these days, and unfortunately the editorial staff is often chosen for cuts.
So there you have it. For every error missed, many more were found and corrected. That mistake that leaps out at you probably represents less than half a percent of the actual errors in the original manuscript. Editors deserve your respect. And I’m not just saying that because I am one. J