It is known from several studies, that students have problems when treating rational numbers. In this paper, there is a focus on rational numbers as equivalent classes, in teaching expressed as extending of fractions. The current research shows how three teachers mediate extending of fractions and how their students understood this concept. The main outcome of the research is that most of the misconceptions were caused by shortcomings in teachers’ subject matter knowledge. For that reason, they were not able to find crucial aspects of the object of learning. Consequently, their students tried to find other, however irrelevant, patterns.