I see this happens in medicine a lot. "The doctor said so". Even if the doctor hasn't updated his knowledge since his college years 20 years ago, even if he confuses "no evidence of disease" as "evidence of no disease", or even if he is a very good doctor, but of an entirely different specialization. A doctor is a human being that can be wrong too! I have also noticed that many doctors get used to this appeal to authority thing and this reinforces their confidence that their knowledge up to that point is really all they need. A good doctor should say "I'm not sure" or "I don't know", but then society does not consider him/her a good doctor.

This happens with other professions as well. People don't like to hear that their expert was wrong about something or doesn't know something. We don't like uncertainty. By the way, in cross-cultural studies uncertainty avoidance has been linked with country/culture. I know its too overgeneralized as a concept but it would be interesting to see if and how that behavior applies in different societies, ie. whether a culture reinforces the "expertise" trend or the "overclaiming" effect more than another.