I miss a word once in a while because I'm typing so fast, to get the ideas out before they fade away. I proofread before posting, but still will sometimes not notice a missing word the first time, because when I reread it, my brain knows what words were supposed to be in that sentence, and supplies the missing one as I skim right over where it should be. Only after it's posted do I notice it missing, probably because the formatting and colors are all different, and its more like I'm seeing it for the first time.
What I really hate is misspellings. With SpellCheck, there's little excuse for those buggers getting past me, but once in a great while, one does.