You key words wrongly rather than mis-spell them. Something to do with the brain not working at the same speed as the fingers. You tend to do it more when typing very fast, to deadlines. I think one of the most common ones is teh and aslo as someone mentioned. I can't remember what mine were, as I no longer have to key in at any speed so don't seem to do it as much.
I remember years ago being really embarrassed when having to type out a Court report every morning and putting in the offence of having possession of a shoot GUN rather than a shot gun. I was only young, so that embarrassed me, particularly as the Seargeant was standing behind me at the time telling me what to type. Nowadays I, nor anyone else would be embarrassed for such a mistake!