This study on incompetent people came out a few years ago but a recent experience reminded me of how true it is.
So, in case you missed it the first time around...
"There are many incompetent people in the world. But a Cornell University study has shown that most incompetent people do not know that they are incompetent. People who do things badly, according to David A. Dunning, a professor of psychology at Cornell, are usually supremely confident of their abilities -- more confident, in fact, than people who do things well. One reason that the ignorant also tend to be the blissfully self-assured, the researchers believe, is that the skills required for competence often are the same skills necessary to recognize competence. "
lots more here: