As with all high level tasks, your brain generalizes simple, component parts (like turning letters into words and words into sentences) so it can focus on more complex tasks (like combining sentences into complex ideas). “We don’t catch every detail, we’re not like computers or NSA databases,” said Stafford. “Rather, we take in sensory information and combine it with what we expect, and we extract meaning.” When we’re reading other peoples’ work, this helps us arrive at meaning faster by using less brain power. When we’re proof reading our own work, we know the meaning we want to convey. Because we expect that meaning to be there, it’s easier for us to miss when parts (or all) of it are absent. The reason we don’t see our own typos is because what we see on the screen is competing with the version that exists in our heads.

Good read over at Wired.

But even if familiarization handicaps your ability to pick out mistakes in the long run, we're actually pretty awesome at catching ourselves in the act. (According to Microsoft, backspace is the third-most used button on the keyboard.) In fact, touch typists—people who can type without looking at their fingers—know they've made a mistake even before it shows up on the screen. Their brain is so used to turning thoughts into letters that it alerts them when they make even minor mistakes, like hitting the wrong key or transposing two characters. In a study published earlier this year, Stafford and a colleague covered both the screen and keyboard of typists and monitored their word rate. These “blind” typists slowed down their word rate just before they made a mistake.
Touch typists are working off a subconscious map of the keyboard. As they type, their brains are instinctually preparing for their next move. “But, there's a lag between the signal to hit the key and the actual hitting of the key,” Stafford said. In that split second, your brain has time to run the signal it sent your finger through a simulation telling it what the correct response willf eel like. When it senses an error, it sends a signal to the fingers, slowing them down so they have more time to adjust.

I'm a touch typist, and I find myself doing exactly what the passage above notes when I make a typo. It's fascinating to understand how that happens.

After a terrible bout with RSI early in my career, I switched over to a Kinesis Advantage Contoured keyboard with my desktop at home and for use with my laptop at work. For ergonomic reasons, the Kinesis moves certain frequently used keys to new places, like backspace, space, delete, and the command/ctrl/alt triumvirate.

However, when I switch back to a conventional keyboard, like the one on my laptop, my brain just subconsciously knows to point my fingers at different spots for those keys. I don't have to think or concentrate, it just happens.

The brain can be an amazing thing.

Keystroke dynamics: “The behavioral biometric of Keystroke Dynamics uses the manner and rhythm in which an individual types characters on a keyboard or keypad.The keystroke rhythms of a user are measured to develop a unique biometric template of the user's typing pattern for future authentication. Raw measurements available from almost every keyboard can be recorded to determine Dwell time (the time a key pressed) and Flight time (the time between "key up" and the next "key down"). The recorded keystroke timing data is then processed through a unique neural algorithm, which determines a primary pattern for future comparison. Similarly, vibration information may be used to create a pattern for future use in both identification and authentication tasks.”
Privacy-enhancing keyboard: A digital keyboard that randomizes the arrangement of its keys so that snooping technology watches where your fingers pressed down on your phone screen can't guess your password.