That the quality of teachers' knowledge has direct impact on students' engagement and learning outcomes in mathematics is now well established. But questions about the nature of this knowledge and how to characterise that knowledge are important for mathematics educators. In the present study, we examine a strand of "Specialised Content Knowledge", SCK (Ball, Thames and Phelps, 2008) of a group of pre-service teachers in the domain of proportional reasoning. In particular, we were concerned with teachers' knowledge of evaluation of the plausibility of students' claims and errors. Our preliminary results indicate that the participants, as a group, had developed a sense of student error but experienced difficulty in explaining the source of these errors.