On my editor page I have mentioned the damage you do to yourself if you are a writer submitting material that was not edited. Silly things appear, similar to the title of this blog, during self editing, fixing typos, changing text, and all the other actions you take preparing your manuscript. You've run a Spell Checker on your file, honest, guaranteed, and yet, they still show up.
It is crucial that the very last thing you do before submitting a file for consideration is to run your spell checker AGAIN. It doesn't matter how many times you have previously run your spell checker, how many times friends have reviewed your manuscript, nor how often you have had critique partners review it. The very last thing you do is to run spell checker again. Unless you can open your file in a view only mode every time the file is opened for any reason it is subject to errant changes.
Those changes may be as simple as hitting the wrong key while trying to scroll down to an area you had an idea about. It could be your family cat streaking by across your keyboard, or the youngster tossed you a ball while you weren't looking. It could be that your sleeve dropped, or your fingers got tired, or you fell asleep at the keyboard. Errors happen, (thanks Forrest Gump).
As an editor, see ( http://bit.ly/9Yk7TX ) I often find problems that could have been found and corrected by the author before it was ever submitted. When editing for a publisher I feel that if the author didn't care enough to run spell check there is no reason for us to care - and the story gets passed over. With hundreds or thousands of manuscripts passing my eyes why would I want to work with an author who couldn't take the time to run spell check?
Another problem that I've seen in manuscripts submitted for consideration to agents or publishers include missing words, or missing sentences, most often caused by cut and paste operations and then the author saves the file thinking their little change is okay as the rest of the document has been edited.
I have read through manuscripts that have been reviewed by other writers, by critique partners, by writers groups, by friends and family, and by other editors, and have always found errors that should have been fixed. You, yourself, have seen errors in published books that made you wonder how it got missed -- and the greatest number of errors could have been caught by a simple spell check program.
And then, sometimes, there are content conflicts. As an example; a recently published book tried to say that four men carried two casket-sized, lead-lined, crates full of gold ingots, along with shovels and picks, across boggy ground in one trip. While editing I noticed the near impossibility of this and had it changed to be more feasible. Yet, making that change required another change near the end of the book, for consistency, and three previous editors had failed to take note.
If a character has black hair you can't let them have blonde hair later without making the change - and yet I find such character adjustments in many manuscripts. Even though spell checking takes care of many problems it can not take care of all the potential problems in a manuscript.
It is important that you get an editor. A real editor. And then send your editor the final spell checked version by running spell check as the last operation you do before saving the file.