I've noticed recently that, when I'm typing fast, I have a tendency to hit d's when I meant to his s's and vice versa. Spell check, of course, finds most of these for me and I can correct them before I've even started the next paragraph, but there are some obvious times when spell check does nothing for me. Do you see it? The problem is verbs.
If I'm trying to type "hatches" and I type "hatched," both are properly-spelled words. Spell check won't notice it, and neither will I unless I see it right away or I happen to go back and read the sentence. And even then, if I'm auto-correcting in my mind, I might not notice the change in tense.
Now, I normally give myself considerable time before going back for final edits, so that ought to distance me enough to find this sort of verb tense error. And I think it's an important one to correct. I don't want editors to think I don't know how to handle the simple difference between past and present tense, after all.
I never noticed myself making this typo before I began writing the novel, and I'll be curious to see how many verb tense disparities I made within 109,000 words.