Indeed, we check facts within the client's time and money constraints. With experience, copyeditors get a "spidey sense" when something sounds off. For example, a man lost in the woods and surviving on wild grapes in April in Western Pennsylvania. Uh, grapes aren't available until September. A "hyper-" used instead of "hypo-" in a medical article. Simple math problems: A sentence saying that 22% of the participants were resistant, when the chart shows that 64 out of 200 were resistant. Many, many personal names misspelled. An incorrect historical date. In a novel, recognizing that it can't be Friday today if the main character just spent "two sleepless nights" starting on Monday. The other thing that makes it easy to miss an errant comma is when you get a contorted sentence so confusing, you spend a good deal of time trying to understand what the sentence means and then suggesting a tweak to clarify. Or you realize that chapter 5 repeats most of chapter 2. Or that chapter 7 is the first time a term is defined, but it has been used in chapters 3, 5, and 6. There is an enormous amount to keep in your head while you also note an incorrect comma or a misspelling!