The ability to offer constructive feedback to students concerning their errors is an indispensable requirement for mathematics teachers, for the purpose of providing cognitively challenging learning opportunities. However, if they are to react adequately, teachers need to identify student errors immediately. The fast perception of student errors can therefore be described as an indispensable part of mathematics teachers’ professional competence. Data on this facet of teacher competence were gathered as part of a national follow-up-study of the IEA’s international TEDS-M (Teacher Education and Development Study in Mathematics) that used a time-limited test to measure teachers’ perception of student errors. This paper aims to provide evidence for the validity of the test interpretation of fast student-error perception as an indicator of professional competence by drawing on contrast groups already used in other studies. Overall, the study could support the validity of the test interpretation because the chosen contrast groups were found to perform either better than the tested teachers—as is the case for the contrast group of mathematicians—or more poorly, as is the case for the group of students. Furthermore, the present study shows that the competence facet of fast student error perception is closer to the domain of teachers’ mathematical content knowledge than it is to the domain of teachers’ mathematics pedagogical content knowledge.