Even more reasons to proofread very carefully:
Missing Words
"I couldn't believe it. I accidentally the whole thing!" Remember, spellcheck won't notice if you drop a word.
Wrong Dates
"My life changed significantly when I started high school in 2080." If you include a sentence like this, your readers will be amazed by either your carelessness or your time travelling abilities.
Verb Tense
Spell check will not have be able to had checking for verb tense consistency, so you must be proofread through the document very carefully before you had submitted.
(Please note: I ran spellcheck on this post. Twice.)
