I don't think that you and I mean the same thing when we're using the word "responsible." I am not using the word in the sense of liability; I'm responding to the assertion above that it's the nurse's job and not anyone else's to make sure there are no mistakes with medication administration. That's not true. Or more exactly, it shouldn't be true. Because nurses aren't any more perfect than the rest of us. So if the pharmacist notices the nurse making a mistake, of course s/he should speak up.
It's not impossible that a pharmacist could catch a nurse's mistake, incidentally. In the ICUs at my hospital, the critical care pharmacists physically come on rounds with us and go over each patient's meds, make recommendations, etc. The pharmacy also has the records of all meds ordered and administered on the floors, so again, they might catch the mistake if a nurse tried to administer a med in a patient's name but no order was written for it by the doctor.