The other day an intern on our copy desk spotted an odd duplication of words on a page proof. Checking the original text, I discovered that the copy editor responsible for it had made edits and had preserved all of the original text, including punctuation, in non-reproducing notes type but had neglected to include one of the duplicated words in the notes face. An easy slip to make.
Trying to read text in the show-changes function of Microsoft Word is bad enough, though often regrettably necessary, but this hybrid notes-text practice is even more challenging to follow, with non-reproducing and reproducing elements jumbled together. (In our system, NewsGate, notes appear in yellow.) It also tends to produce odd effects, such as additional spaces and stray punctuation.
I would recommend this procedure to an intern or a probationary editor, in early days, until some confidence about the editor's reliability developed. But after that, preserving the original text in notes face opens up too many chances of misreading the text or allowing irritating and avoidable minor lapses.
I speculate that when experienced copy editors are compelled to edit in this manner, it is because their shop does not trust them to edit and requires them to show their work. Really, if you are going to hire copy editors and allow them to have their hands on texts, they should be allowed to edit. Most editing software allows for retrieval of earlier versions of texts for comparison; and if such comparison shows the copy editor's work to be unreliable, the editor can be instructed to do better to be dismissed.
I can't recall how many times over the years I have uttered this plaintive cry: Just let my people edit.