These are always held up as the worst thing you can do. Horrific grammar mistakes bother me, but there's no way I'm going to overlook a qualified candidate because they made a typing mistake. I remember one resume I received. I believe we had mentioned that the job involved giving presentations to staff. The cover letter assured me that the candidate was used to "resenting information". I laughed my ass off. What could be better for a library position? And then I interviewed the person.