At Wired, Nick Stockton on why it’s so hard to catch our own typos:

You have finally finished writing your article. You’ve sweat over your choice of words and agonized about the best way to arrange them to effectively get your point across. You comb for errors, and by the time you publish you are absolutely certain that not a single typo survived. But, the first thing your readers notice isn’t your carefully crafted message, it’s the misspelled word in the fourth sentence.
Why?
The reason typos get through isn’t because we’re stupid or careless, it’s because what we’re doing is actually very smart, explains psychologist Tom Stafford, who studies typos of the University of Sheffield in the UK. “When you’re writing, you’re trying to convey meaning. It’s a very high level task,” he said.
If you’ve been following Daily Exhaust for a while, you’ve likely seen many of my typos. I always miss them. Luckily my friend and DE contributor Bryan always spots them like a sharp shooter.
Ironically, I notice I’m usually able to spot my typos after I hit the Publish button and I’m reading my post live on the site. I attribute this to my brain leaving the “conveying meaning” mode (see above quote) and switching into “reading” mode. I’m no longer close to my words.