I find I do that when my speed of thought outpaces my rate of typing. I "know" the words are there in that they're part of the sentence I intend to type, but I'm not closely enough attending what my hands are typing to notice they didn't come out.
Worse is when you spend a bunch of time rewording an email to upper management or an important stakeholder making sure they don't get the wrong idea or misinterpret your words, re-read it several times, hit send, then check your sent items and notice words missing :')
I wonder if proofing it by ear via TTS might make stuff like this easier to catch. The "come back and reread in half an hour" method doesn't always, unfortunately.
Read it backwards. Start at the end, and read word for word, just in reverse order. This forces you to slow down enough that you end up processing single words rather than sentences and phrases. As a result, you can notice misspellings and missing words more easily than while reading normally.
This is why we proofread?
Eh. I'm sure this isn't the only place this author writes, and I suppose Stalin's maxim may apply.
incidentally, chrome has an integrated proofreader now! i was pleasantly surprised when i saw the feature. doubly so when it said it's the same one gmail uses.
What's Stalin's maxim?
Oh, the old bit about "quantity [having] a quality all its own".