I ask a lot of people this question and they don't notice. This is why a lot of people don't notice mistakes in books. The eye fixes the mistakes automatically and moves on... unless it's really glaring. I guess that's good. But when you're an author and you're trying to edit your own work, it's bad. You don't want to miss something! And believe me, it's really easy to do. This is why it's very important to have several people who don't know your story read through it. Trust me. You'll be glad you did!