We all slip. I know that. I often type "withing" when I mean "within" because I type really, really fast (80-90 wpm when I really get going...around 50ish in general) and my fingers are so used to the -ing pattern that they just add it without my brain's conscious permission.
(And, yes, typing is an unconscious thing for me; I don't spell words as I type, but I do hear them in my head -- I know there's an educational term for that, but I don't recall it at the moment.)
Anyway, I know we all slip, but slips gall me more when they are made by teachers or other school-related folks.
So this just boggles the mind.