Knowing a lot about a little and then thinking we know a lot about everything is not uncommon in experts. Aside from making a person very boring it takes away their ability to grow, learn, solve new problems, take on new challenges or otherwise expand their horizons.
The expert mantle also takes a lot of the fun out of life if you can't do anything that might make you look foolish, silly or stupid. Experts take pride in their ability to master something and get paid for it, but then may be too fearful or self-conscious to ever look like a beginner again.
Thinking we know more than we do in the wrong circumstances can cause real problems. Just because I'm an expert in say Molecular Biology it doesn't mean I'm an expert in wiring a house, advising people on taxes, recommending investments, or installing a gas hot water heater. We have electricians, accountants, and plumbers who are expert in those things - but we all know someone who is expert in something who's perfectly happy to give out advice on house wiring, taxes, investments or installing gas hot water heaters. They may actually have something worth sharing if they are amateurs who have done these things, which they are unless they are Molecular Biologists who moonlight as paid electricians, accountants or plumbers - but we could all lighten up a little on the expert mode.
There's another issue at work here in that people who are Molecular Biologists (for example) sometimes give the impression that, because they have a degree and have studied and work in an academically challenging domain that makes them somehow "smarter" than a waitress, cook, electrician, plumber, farmer, etc. Not only is this a bad assumption it also makes it hard for them to learn much from others because they can't listen or they shut down other people with their attitude.
As long as we have some experts (people who know all they need to know in their specific area) but hopefully more beginners (people who are pretty sure there is a lot to learn) society will continue to evolve.
Life is always providing us with something new to learn - if we just look with a "beginners mind".
This is a quote from the August 10, 2009 Time Magazine article The Avenging Amateur by Kurt Anderson -
"...frankly admitting that we aren't absolutely certain how to proceed is liberating, and crucial. I like paradoxes, which is why, even though I'm not particularly religious, Zen Buddhism has always appealed to me. Take the paradoxical state that Buddhists seek to achieve, what they call sho-shin, or 'beginner's mind.' The 20th century Japanese Zen master Shunryu Suzuki, who spent the last dozen years of his life in America, famously wrote that 'in the beginner's mind there are many possibilities, but in the expert's mind there are few.' Which sounds to me very much like the core of Boorstin's amateur spirit. 'The greatest obstacle to discovery is not ignorance,' Boorstin wrote, 'but the illusion of knowledge.'"
The specialist says - I learned more and more about less and less that I finally knew all there was to know about nothing. The generalist says - I learned less and less about more and more until I finally knew nothing about everything. Someone in between there's a happy medium.