The average reader can only take in six letters accurately with one fixation. This means you have to fix your eyes on almost every word you have written and do it twice in longer words, in order to proofread accurately. You have to look at the word, not slide over it. In proofreading, you can take nothing for granted, because unconscious mistakes are so easy to make. It is often possible to hear a mistake, such as an omitted or repeated word that you have not seen.
Professional editors proofread as many as ten times.
Publishing houses hire teams of readers to work in pairs, out loud. And still errors occur.