In 2008, we decided to investigate error learning processes in German vocational schools. Our focus was on accounting education because accounting is a central topic in commercial vocational education, and it is known to be error-prone. We therefore wanted to learn more about how well teachers diagnose student errors and how they react after identifying an error. To answer these questions, our first instinct was to observe classrooms. However, we found that you have to observe for many hours to find student errors and teacher reactions. One reason for that is that teachers see themselves as conveyors of knowledge, who reveal factual knowledge step by step and present it in small parts by asking many questions. In this setting, errors other than minor slips and lapses are rare. Severe errors are usually hidden below the surface. Therefore, we changed our approach. We decided to interview experienced teachers, who had been teaching for many years and were familiar with typical student errors, to learn more about errors that have the potential to support learning processes. We wrote a script around these errors, filmed typical error situations, showed these video vignettes to (prospective) teachers, and asked them how they would deal with such situations. This case study is about the development of video vignettes, and it sheds light on the challenges of developing and using video vignettes to assess aspects of professional competence of teachers. The results show that video vignettes have distinct advantages over other methods of competence measurement.