An important aspect of reform-based mathematics teaching is how students’ errors are handled in the classroom. This study aims to investigate the relation between mathematics teachers’ beliefs, mathematical knowledge for teaching (MKT) and error-handling practices in the classroom. Using an abductive approach based on 51 lessons with 12 teachers in Grades 4–6, we analysed 469 episodes containing error-handling practices. Patterns in the 12 teachers’ error-handling practices were found in relation to their MKT and belief profiles. The most important dimension in the patterns found is that teachers’ MKT seems to determine their possibilities to act according to their beliefs. That is, the results of the study show that high MKT makes it possible to handle students’ errors in accordance with the teacher’s beliefs (regardless of whether or not they hold more reform-oriented beliefs), while lower MKT seems to constrain their possibilities to explain and discuss students’ errors in conceptually grounded ways and act in reform-based ways.