When all the characters speak in the same way, and so does the narrator. Too many examples to list (plus, I don’t want to upset or get into an argument with anyone. If they don’t know, they don’t know). Yes, writing in voices is harder than it looks; but there’s no excuse for not doing it.

[On the other hand, let’s remember that a few years ago I read a prime example of this sort, described on its front cover by a Famous Author as “a great book”. “No”, I said, "this is awful, I could do better myself" . So far I’ve written a novel and a half, and a lot of other stuff besides. So, thanks for the inspiration, Unaccountably Published Person.]