You read that email like 30 times before you sent it. How did that typo survive the scrutiny? Nick Stockton explains for Wired what went wrong in the brain.
Lousy, Useless Brain
Basically, when we write/type, our major focus is on conveying the big picture ideas, and not so much on the details that make up the picture. Since we know what we mean to say, our brains fill in any gaps when words we think are there are actually missing. Such generalization is often necessary though in order to swiftly process all the information we are giving and receiving. Interestingly though, anyone who can type without looking at the keys usually recognizes that he or she has made a typo before the mistake even appears on the screen, due to the person having a mental map of the keyboard and which key is right to hit. These instincts do not translate to grammar though, so it is recommended that if you really want to catch typos, you need to change the document’s appearance on the screen. Change the font, change the color, and maybe even print it out and examine it by hand. The important thing is that the document looks new and foreign, to stop your brain from filling in gaps.
You can read the original article here: http://www.wired.com/2014/08/wuwt-typos/