Abstract
Didactical content knowledge is concept frequently discussed in recent didactical literature. From this wide field we focus on one of 20 parameters which characterise teacher's educational style.
Perspective on teacher's didactical abilities we narrow on important area of teacher's work with pupil's mistake and we describe 4 didactical mistakes, which we illustrate by written test of five student teachers. We try to reveal causes and consequences of those mistakes and find such reeducational tool, which would enable the teacher to overcome the mistake.