You need to have some feeling for why the mistakes are made. I have dyspraxia, better known in the past as clumsy child syndrome.
The problems it causes were best summarised by Morecambe and Wise, in their quip about one of them playing the piano 'All the right notes just not in the right order.
My head is an excellent speller, no problems with punctuation or syntax. Unfortunately there is a disjuncture between what my head is quite clear about and what my fingers would prefer to type. I can say out loud as I type - 1- 2- 3-4. My fingers will still type 1- 3- 2- 4. It follows, as night the day, that proof reading, as I usually try to do just clears most errors, only to introduce more.