I find the inclusion of correctly-spelt but incorrectly-placed words (where/were is my usual bugbear) to be the greatest problem for me.
If it's something longer than a sketch, and something you've worked-on for a while, it's easy to skim it rather than read it word by word and so some strange things can get left in. Good to leave such a piece for a day or more unread (if you can spare the time) and return to it to spot any cock-ups like that.