Over at Poynter.org, Craig Silverman writes about how "good copy editors are 'abnormal' humans." Most people already knew that; just look at the photo of the peculiar wretch he picked to illustrate his article.*
What he means by abnormal is that effective copy editors can disable the "auto-correct" function built into the way our brains work. In that function, "we see things that aren’t there and miss things that are."
The reason for this is that the brain is not a passive recording device but an active processor of stimuli. We've begun to understand this about memory. The brain is not a video recorder of experience but an interpreter of experience. Our memories are the stories we tell ourselves about our experiences. That's why eyewitness testimony is notoriously variable; each witness processes fragmentary perceptions and constructs a memory from them.
Something like that happens in reading. Images are transmitted to the brain through the optic nerve, but the brain interprets those images. This is why experienced readers, the very people you would imagine to be effective at editing, are not necessarily reliable. Experienced readers recognize patterns. They recognize patterns of letters arranged in words. They recognize syntactical patterns. They recognize stock phrases and standard figures of speech. And so the brain tells them that they see the patterns they expect to find. The brain auto-corrects, not worrying about, for example, transposed letters, because it recognizes the shape of the whole word.
Experienced readers are like James Thurber and his friends who invented a game called "superghosts," like the word game ghosts but with the refinement that a player can add a letter to the beginning or the end of a proto-word. Success, Thurber writes, depends on things like recognizing that "cklu" is the guts of "lackluster." He also says that the game tends to end in a lot of shouting.
The trick in editing is to suppress the auto-correct without entirely turning it off. If it were turned off, the editor would have to read slowly, painfully, letter by letter, like inexperienced readers. The editor has to maintain a minimum speed, but slow enough to spot the transposed letter, the misplaced homonym, the absent (or superfluous) punctuation, and all the other hazards. To be effective as an editor, one must train the brain in the editing function, which, like reading and writing in the first place, does not come naturally.
What assists the editor, making the enterprise possible, is, once again, a recognition of patterns. Experienced editors know what the most common errors are, and are sensitized to them. Really experienced editors, like those at a newspaper who work regularly with a group of writers, come to know the lapses that each writer is prone to, and watch for them.
We know who you are. We saw what you did.
*And here's the Guardian article on the workings of the brain to which he refers.