I think some spellcheckers are also to blame. Every word processor has a spell/grammar checker and sometimes they insert something or automatically change something while you're writing and you don't even notice until you proofread. You can turn some of these functions off, but I find them quite useful most of the time.
I wrote a story spelling the word "pervert" as "prevert" to make a specific point, and of course the spellchecker automatically corrected it. I had to make sure it was spelled the way I wanted it because the whole point would have been lost had been spelled correctly (it was a boy's misunderstanding and mispronunciation of the word that was at issue).
I'm sure everyone here does the best they can, but some misspellings are especially egregious, which leads me to believe that indeed some word processor functions are to blame more often than not. I'm pretty sure my story is otherwise error free, but I only proofread it three times...

btw, ordalie, nice smilie. I saved that one for my own use...