Controversial issues characterise life in democratic societies and they often arise unexpectedly in the classroom, without being planned for by the teacher. However, controversial issues are rarely addressed beyond a mandatory curriculum and are often avoided. The aim of this exploratory study is to investigate what teachers identify and address as unplanned controversial issues in the classroom and the content of such issues. Semi-structured interviews were conducted with a purposive sample of 12 teachers from primary, secondary and upper-secondary schools across Europe. Unplanned controversial issues identified fell into three categories (1) mainstream controversy, (2) teacher-initiated controversy and (3) controversial pedagogy. Mainstream controversies follow mainstream definitions of controversial issues. In teacher-initiated controversies, teachers reacted to statements made by students, taking a stand against examples of prejudice, stereotyping and discrimination. Controversial pedagogy refers to instances in which the teachers' behaviour was deemed controversial by the teachers themselves. The content of the unplanned controversial issues that teachers identified and addressed is multi-faceted and draws upon personal, current, cultural and historical dimensions. The findings suggest that more attention needs to be paid, among other things, to the political dimension of education, teacher vulnerability, and who the person in teaching is.