As a writer of fiction, I'm often frustrated by copyeditors who place strict adherence to arbitrary and often artificial grammatical rules over good writing -- for instance, replacing a perfectly smooth and euphonious construction with a painfully awkward one just to keep a sentence from ending in a preposition, or harping on a totally imaginary rule (actually just a suggestion some grammarian made that later style guides somehow chose to interpret as an absolute doctrine) about when to use "which" and when to use "that." What particularly bewilders me is when they do it in dialogue passages, as if it were reasonable to expect people to use perfect, formal speech all the time. Sometimes they harp so much on grammatical and structural precision in dialogue that they bulldoze over intentional choices I made to convey character and emotion. It takes a lot of work to fix such "corrections."
This is surprising, especially in mass market fiction as opposed to, say, nonfiction at a university press. Everywhere I look I see the old, picky rules violated; I thought no one cared.