"He who knows best knows how little he knows." -- Thomas Jefferson.
The Dunning-Kruger Effect: Incompetent people systematically overestimate their abilities and believe that they know more than others who are far more competent.
The term is named after Justin Kruger and David Dunning whose Cornell University study was published in the Journal of Personality and Social Psychology (December 1999): "The Unskilled and Unaware of It: How Difficulties in Recognizing Ones Own Incompetence Lead to Inflated Self-Assessments." Their work follows from the research of others, such as W. I. Miller who observed that "it is one of the essential features of incompetence that the person so afflicted is incapable of knowing that he is incompetent" (1993).