Another commone error is when a person has expertise in one subject area, and then others assume that this expertise carries over to other domains. A prime example is celebrities, who often dispense advice about diet, love, and who knows what else. Unfortunately, their true expertise lies in acting, and not all these other subjects.
Or for another example, I have a Ph.D. in Economics. Does that mean I know anything about health? That's up for readers to decide. But I don't assume that my expertise in one field carries over into another.
You also run into this error with M.D.s, as well. For example, lots of people criticize Dr. Dean Ornish. They say he's this medical doctor who doesn't really know anything about nutrition. Yet if an MD supports a Paleo Diet, then all of a sudden he or she has credibilty. Why is one person's MD credentials important now but not the other?
Credibility lies in knowledge, and not necessarily academic degrees.