It happens to everyone. You proof your work. You read it and re-read it.
But unless you put a finger on every word (nearly impossible with touchscreens) you won’t find those missing/wrong words.
There are some tricks to self-proof reading.
#1 is probably most important.
1. Print out the book. The look and feel of a print-out is totally different than a computer screen.
(you might want to pay for the printing rather than use expensive ink. Some printers will do this overnight for 2.5 – 3.5 cents a page.)
2. Edit after letting the manuscript sit for at least two weeks.
3. Only edit 20 – 40 pages at a sitting.
4. Read each paragraph twice!
5. Go back and re-edit those same 20 -40 pages, looking more critically at the same words.
6. Know you’ve missed something – look for it.
7. Go slow. You want to find the mistakes, not get done. You want to read each word as it is written, not what you think is there.
8. Reading out loud is one of the best way to proof read. Can’t do it in the library.
9. Do not trust your spell check – ever.
10. Proof reading is not revising. These are two different processes. Proof after you revise.
Finally, when you are done and you know it’s perfect, if you are considering sending your manuscript to an editor or an agent, hire a professional editor.