Competence: Overestimating Happens
People, especially Americans, tend to overestimate their own competence, according to social psychologists. (Via Human Nature Daily, whose name overestimates their own publishing frequency.) Ironically, the worse someone actually does, the more likely they are to overestimate their performance.
While we're on the whole subject of incompetence, women tend not to buy excuses for poor performance from men (or women, but the article focuses more on the male-female side, presumably to add war-of-the-sexes drama.)