In previous papers we proposed a didactic method for the active learning of greedy algorithms, that comprises an experimental method supported by the GreedEx interactive system. The didactic method has been used and evaluated in a course in several years. As a part of this evaluation, we have analyzed the reports delivered by students in three consecutive academic years. In this paper we present our findings, including several unexpected misunderstandings of students. We also describe the interventions addressed, resulting in the removal of the most severe misunderstanding. The main contributions of the paper are two-fold. On the one hand, we identify a set of misunderstandings about greedy algorithms. On the other hand, we present a case of action research, successful thanks to an adequate combination of: didactic method, educational software, scheduling of classes and lab sessions, and development of teaching materials.