Short answer: Your eyes will deceive you.
Spelling will be our first discussion.
Somewhere around the second grade I began writing the word friend as FREIND. It was sixth grade before I broke the habit. As a writer, an editor, and an average American with a good grasp of language, I still pause every time I write the word. And I’ve been out of elementary school for a few decades. My brain knows the correct spelling. My eyes know it. But having started telling stories though written word at a young age, and spelling one of those words incorrectly for a long while, FREIND became normalized.
When writing by hand, in that oh-so-archaic way of pencil and paper, your brain works faster than your fingers. Errors in spelling are easier to detect, often while in the middle of writing the word in question. Erase, swipe, correct.
But our culture does not rely on pencil and paper much anymore. I’m speaking to the world larger than elementary school. We blast away at furious finger speeds on computers, tablet pads, smart phones, and in most cases we are using some fancy program that fixes our mistakes for us. This is great! Or is it… ? I had to incorrectly spell FREIND three times now, and each time my computer corrected it for me.
So while our gadgets are correcting what our brains should have learned years ago, we are continuing to reinforce in our grey skull-nuggets that we can be lax in spelling, punctuation, grammar. We’ve taught our eyes that if a squiggly red line does not show up under a word, the word is correct. We get irritated when the squiggly green line shows up and doesn’t explain to us why there is a syntax error. But the real problem is when those squiggly lines don’t show up.
You are with your piece of work from its inception, clearly. You are the writer, the author, the creator. (Settle down, ego.) You pound on your keyboard as fast as you can, because no matter how long you have been an author you know the brilliant inspiration that grabbed you a moment ago may… Wait… what was I going to write?
Errors are more likely, and more permissible, in this flurry of impatience. Flexibility begins to tighten at each stage of editing. On your first fresh read-through you are going to catch a few goofs, but the computer will have cleaned up your spelling pretty well. With subsequent edits, it will be the words that are correctly spelled, but incorrectly used, that will slip by. Not because you are a nincompoop when it comes to spelling and grammar, but because your eyes will have grown accustomed to seeing the error as part of the piece. Freind will have become normalized.
Its the simple errors that will be the most difficult to detect after a fourth read-through. Did you catch that? Its should have been It’s. Really basic stuff, I know I’m not teaching you anything new. But I am pointing out the obvious reason an editor is a writer’s friend.