That, of course, is exactly the kind of error editors are supposed to catch. I hadn't. Worse, when I went to correct the error, had to be reminded by the author I'd also have to change "brother-in-law" to "son-in-law" throughout.
Why I hadn't caught that, I'll never know. Lack of sleep, I suppose, or rushing, or my faith that this was one of those very few authors who didn't make mistakes. (Also, author's spouse is incredibly good copy editor, so usually submission is pretty clean before it leaves the author's hands.) But still, I'm pretty embarrassed.
Fortunately, we caught it before it went to layout (i.e., typesetting). Would like to think it would have been caught at the Galley stage anyway, but the sooner one can eliminate errors, the better.
Of course, mistakes are inevitable. If an editor claims they never let anything slip through, they're lying.
Why there are galleys. Even when editors catch mistakes--say, notice that the spelling of a minor character is different in different parts of the book--it often turns out that was the mistake: there were two different characters with similar names. You know? So authors have to be given opportunity to read through the book one last time before it goes to press. And even then, there are always things that slip through. Usually, right there, front and center, at any random page you first open the final published book. *Sigh*
The worst mistake I was (tangentially) involved in was back when I was working for the government as a Test Developer (i.e., test writer/editor). A Grade 12 Mathematics examination went out with one of the equations missing an exponent, which meant that there was no correct answer for that multiple-choice question. My boss was assigned to find out what went wrong, the implication being that heads were about to roll.
He traced the error back to the original manuscript, but the question wasn't why the author made a mistake (everybody makes mistakes) but why it wasn't caught in all the checks that were built into the process. The provincial exams were the most heavily reviewed and edited documents on Earth, so how did the error get through?
When my boss followed the trail of signatures, over 36 people had signed off on the document. 36 people. I was one of the Test Developers who had reviewed the test, but I pointed out that as a Social Studies guy, I had no clue about the actual math (hey, it was the advanced Grade 12 course. Calculus. I had no idea what the test was asking, let alone what the answer might be.) My job had been to just proof for format and so on. It had checked out on that. But even allowing that some of the reviewers were checking for other stuff, it's still astounding that with over 25-30 people actually checking for content, no one caught such an obvious error.
After much hand-wringing and discussion of how to rectify for future, management cut the number of people reviewing the examination to five; one each at each distinct step. Because it turned out with 36 people, each reviewer was subconsciously thinking, "I can do this fast because if there was anything, one of the others would have caught it by now." No one could be held responsible because there were too many people to share the blame. But with five checks, one at each stage, someone was specifically responsible for it being perfect at that stage. Actually turned out to work a lot better. Another example of less is more.
So the moral here is, it's better if your editor starts from the assumption that you're an idiot. That way, they'll check everything with obsessive care. Thinking you might actually know something about punctuation, Canadian spelling, consistency and so on just leads to errors.