Obviously, this has happened. But it is extremely rare in these days of universal precautions, and needle-less IV systems. Considering that there are millions of opportunities during patient contact, with appropriate care, this should not be an undue cause for concern.
I'm in nursing school and am allergic to an ingredient in the Hep B shots and cannot get them, so I had to sign a waiver. I follow all universal precautions to the T, even if it's overkill sometimes. I know I am not alone in not having the shots, but it is always better to be safe.