The conceptualization of teachers' professional competencies has evolved in the last decades. The specification of the types of knowledge that teachers require, the inclusion of affective and motivational aspects and the acknowledgments of the processes that connect these dispositions to the behavior of teachers in the classroom are considered especially important. In particular, teachers' diagnostic competence has been regarded as crucial for successful teaching because it allows teachers to understand students' thinking and make a corresponding plan to promote learning. Errors have been recognized as a valuable source of information about students' thinking and therefore, teachers' diagnostic competence in error situation is the focus of this study. This article shares the design of a university course and the theoretical basis aimed at developing pre-service primary school teachers' diagnostic competence in error situations within their initial teacher education programs. Its implementation in Chilean universities suggests a valuable opportunity for future teachers to learn and discuss about mathematics and its teaching and learning.