I've heard some mightily staggering tales of incompetent or presumptive copy-editing. Of course, I heard them from the authors. Except in one case where I was the author's research assistant, and saw it for myself. It was pretty amazing, including the nonsensical "correction" of grammar within quoted material.

Frank M. Robinson once delivered to me a discourse on what editors hate about writers, and then turned right around and delivered an equally pungent discourse on what authors hate about editors. He, of course, had been both.