Believe me, I understand typos. Even though I obsessively read each blog post two or three times before publishing, I know the occasional typo sneaks through here. There are situations, though, where they should not exist. Among these are professionally published books. Nothing takes me out of fiction more than having to mentally reconstruct what the narrator meant to say, and nothing is more useless than a textbook that obscures the facts it's trying to teach in language that forces the reader to guess.
The situation where typos are unacceptable are in teacher feedback on class assignments. I'm not talking about simple misspellings here, where the student knows exactly what the teacher is saying, but rather the exclusion of key words that completely change the meaning of your intended message, like saying "You have included discussion of the Keller’s ARCS model of teaching model" when you meant to say "You have not included discussion..." (Notice I'm not complaining about the redundant model here.) Besides confusing the student, such thoughtless oversights send a clear message that you are rushing through the grading process and not paying much attention. I understand that it takes a long time to grade--I decided not to be a teacher largely because I got burnt out from grading 80 12-page papers in the course of a week--but that's an inevitable reality of teaching. It sucks, but you're the one who chose the career. If you'd given my paper just a little more attention, you might not only have missed that typo, but you might have noticed that I did in fact discuss Keller's damn ARCS model of teaching model.