Proofreading is as important as writing
This somewhat tongue-in-cheek blog posting illustrates the importance of proofreading your work before sending it off. Regardless of whether it's an email, a blog post, an article, a book, or just your homework.
A while back, I wrote a blog post that quoted William Zinsser. He got it right when he wrote that just because you're writing fluently doesn't mean that you're writing well.