The article is an endeavour after the classification of pupils’ mistakes and a survey of some of their causes. The author classifies the mistakes of pupils in this way: The mistakes made by chance and those repeated regularly, either individually or in common. The first area of the causes concerning repeated common mistakes refers to the pupil as a subject whose response is different from what he has been expected to give, i. e. a response in a paradoxial way. The second area of the causes is in not proper didactic procedures, the third one concerns the mistakes made by the author of the textbook or by the teacher. It is possible to prevent the series of repeated mistakes of pupils so as to draw attention to the danger of a mistake, or so as to discuss the cause of it and to introduce a number of exercises according to the probability of how often the repeated mistakes occur.