I had an editor during my stint as a tech writer. She could find things like, "On page 05-4 you refer to the process as XXX, but on page 14–3 you refer to it as XYX. Which is it?”

She would circle periods at the end of sentences, in the printed version of the drafts. “What’s wrong with the period?” “It’s bold.” “WHAT?!” I’d go into the Word document and yup. Little speck of toner at the end of the sentence was, indeed, bold. <shiver>

Her OCD meant she couldn’t share the office candy dish because “other people had put their hands in there,” but as an editor, oy gevalt, she was scary good. Different flavor of editor than usually works in fiction, but I bet she could have made sure the Harry Potter books were all consistent.