Finding and fixing errors is an essential skill in learning programming in the K-12 classroom. However, most of the time, debugging only plays a minor role in teachers’ approaches to conveying programming - especially as they themselves rarely learned debugging explicitly and lack appropriate concepts and content. In consequence, students often struggle with finding and fixing errors on their own. Professional development allows for disseminating research findings and corresponding teaching materials to eventually influence the teaching practice. In this paper, we present a professional development workshop and its theoretical foundations, aiming at fostering teachers’ professional competence with regards to debugging. We investigate changes in teaching practice and the teachers’ beliefs in reaction to the PD using a case study approach. The results provide insights into impact and effects of professional development with regards to debugging in the classroom. Furthermore, our study contributes indications for designing professional development that fosters actual change in the classroom.