Recent discussions about competence development of (future) teachers list “catalogue” of competences that are considered to be relevant for acting as a professional in the classroom. Shulman (1987) suggests four categories: general pedagogical knowledge, subject-matter content knowledge, pedagogical content knowledge and curriculum knowledge. Unfortunately the various facets of competences that can be subsumed under these categories are neither precisely defined nor studied. This is as well the case when we focus on teachers’ competence to diagnose students’ errors in learning processes and to constructively deal with them. In our study we therefore try to shed light on these questions. 1.136 students were asked about the “error climate” at their schools (teachers treat errors as deficits and sanction them vs. errors are seen as a chance to learn). From this sample two classes were videographed (10 classroom hours). Errors in the learning process were identified. We then analyzed in which ways teachers tried to clarify these errors and what kind of feedback they gave their students. Findings so far suggest that the climate is generally good but that teachers do not really know how to give professional feedback and how to use errors constructively for subsequent learning processes.