It would be absurd to say that I was born a copy editor, but the genes certainly seem to run in the family. My mother was an English teacher before she married and always wrote and spoke very carefully, my daughter is a gifted proofreader, and even my son, though lackadaisical in his own writing, notices errors in published works.
This ability is a mixed blessing, of course. When the one error on a written or printed page is the first thing that jumps out at you, especially if you can’t resist correcting it, you don’t always have a lot of friends, and “reading for pleasure” can sometimes be downright painful. But copy editors perform a useful service (though there seem to be fewer and fewer of them doing it, judging from the published books I read), and every writer should be grateful for the help they can provide.
I was reminded of this (as I often am) this morning by seeing the moon, a few days shy of its last quarter, high in the sky as I was walking. According to the moonrise/moonset table for my location available from the U.S. Naval Observatory, the moon rose last night at 10:56 and will set this morning at 10:13. Anyone who wrote a book and set a scene on the evening of June 22, 2008, under a full moon would obviously be mistaken. Although a copy editor is supposed to check on such things and alert the author, both of them know that few readers will notice or care. But if you make the moon full tonight and then make it full again two weeks from tonight, some reader probably will notice that anomaly.
Protecting authors from that sort of blunder is part of what a good copy editor should do, which is why it’s often difficult for a copy editor to render an opinion on the book in general. “Well, what did you think of it?” is a question I dread, for I often have no overall impression, just a collection of “things that need to be fixed.”
Errors in chronology can be among the most difficult to deal with. If the author has been vague about dates, these will not usually be an issue, but if there are even a few specific dates to hang events on, then every other event in the book must be evaluated in relation to those set points. I think perhaps the worst problem I ever encountered was with two children whose ages, it was clear from later context, were supposed to be separated by a year or two. Yet careful reading and calculating the timing of events showed that they had to have been born about three months apart—to the same woman!
Errors of fact should be easier to avoid. The Internet has made research so easy that there is no excuse for misspelling a brand name, attributing a quotation to the wrong person, or introducing anachronisms. The same writer who created the prodigiously fecund mother also had one of his characters flying a company Learjet—in 1960. Unfortunately, according to Wikipedia, “The original Learjet 23 [the first model] was a six- to eight-seater and first flew on October 7, 1963, with the first production model being delivered in October 1964.” I suggested the Grumman Gulfstream I, a turboprop model that was a popular corporate aircraft during the required period.
The hardest part of being a proofreader/copy editor, however, is the knowledge that all your friends will gleefully gloat over any mistake you make—and these are inevitable. Because an immutable fact—and the reason even the best writer needs a proofreader—is that it is impossible to proofread your own work (because you know what you intended to say, and so that’s what you read, regardless of what’s on the page). I say this in advance, knowing that, according to whatever corollary of Murphy’s Law governs such things, any piece of writing that points out others’ errors is bound to contain an error of its own.
WalkThoughts 
“I say this in advance, knowing that, according to whatever corollary of Murphy’s Law governs such things, any piece of writing that points out others’ errors is bound to contain an error of its own.” That law, in fact, is Muphry’s Law [sic]; see https://en.wikipedia.org/wiki/Muphry%27s_law.