As a writer I know that I have to proof my work, however, after reading the same piece over and over again your eyes get tired and you can overlook the most of glaring errors.
I read right over a your that should be you’re, a their that should be there and more hads, haves, and has’ that should not be in the text at all. I’ve used the same descriptive verb in back-to-back sentences and in one case, had the wrong character speaking dialogue. (That’s what happens when you write late at night.)
I’ve been embarrassed by my beta reader finding my error, followed up by the editor who found errors in my corrections and the proofreader who found errors none of the others picked up. However, the ultimate faux pas is to have your errors picked up by a reader, someone who paid for your book. And, to my chagrin, that has happened in one of my novels. (I won’t tell you which!)
So now you know the book you read goes through many hands before it gets to yours, each one trying to make it as perfect and error free as possible. But remember, even spell-check sometimes gets it wrong!