Scope of Practice
Maybe you've seen the commercial on TV recently that features all sorts of kids doing a wide variety of activities and hobbies. The point and catch-phrase is, "No one's good at everything, but everyone's good at something." It's a great campaign to promote self-confidence in young people and to point out that you can be an expert in pretty damn much anything.
When you get into a specific field and study it all the time, you can become an expert. That happens most commonly with advanced education, to the point where people get Ph.Ds and lead cutting-edge research and development into new areas. It's inspiring.
The danger with higher education and advanced study is thinking you are an expert in more than your field. For example, I'm a massage therapist; I know more than the average Joe on the street about anatomy and physiology, but I certainly wouldn't call myself an "expert" by any real standard. You want an "expert", call an MD or preferably an anatomist. That being said, I tend to be the go-to fella with any medical question amongst our friend group because there's no one more qualified that we drink beers with.
Massage therapists have a limited scope of practice in that we work on soft tissue (muscles, ligaments, tendons) and joints. Our main goal 90% of the time is relaxation, whether it is for stress relief or therapeutic effect, so the actual massage doesn't vary too much. Sure, there's the odd joint mobilization or friction, but generally, our focus is quite narrow.
We step outside our scope when we say that massage can treat, say, MS or Parkinson's or cancer (the actual conditions, not the symptoms, which we can treat to a small degree). Going outside your specialty is a no-no and often gets a pass amongst lay-persons going for advice, or, more commonly, TV interviewers asking someone who has no business commenting for their opinion. You've all seen the "Man On The Street" segments where a reporter asks Julie, the McDonald's girl, what she thinks of the election results in Iran and what the government should do.
Now, I'm not saying that Julie the McDonald's girl is stupid, I'm not saying that she can't have an opinion, but does anyone care what she has to say? No one should care what I say about (anything, really....) economics because I know less that shit about economics. I have opinions, but they're uninformed and probably retarded. If I want to hear someone talk about Iran, the economy, NASA and the space program, or how to build a tree-house, I'd like to have someone who actually put some damn time into learning about that specific thing.
A few years ago, I went to a series of young Earth creationist (YEC) lectures. One of the speakers was a man named Bruce Malone who said he was a Ph.D in chemistry, but if you read some of his articles (like this one) over at Kent Hovind's site, you'll see that he dips his toes into evolutionary biology, astronomy, and geology - subjects that he has, at least in the case of evolution by natural selection, exactly zero grasp on.
Prime example of going well beyond your scope of practice/expertise. The thing about Malone is that he even screws up when he talks about his supposed area. I find that some of these people just can't do anything correctly. Look over at Orac's blog when he exposes Dr. Jay Gordon as a dumbass when Gordon should be an expert in vaccinations but chooses to keep his head firmly embedded in the sand of ignorance (otherwise known as Jenny McCarthy's cleavage). It's a wonder to behold.
So the lesson to be learned is to be humble about your expertise, but not so much that you let people who have only strong opinions (or a damn "mommy instinct") lure you away from science-based practice.