Often, students repeat the mistakes they have made in answering exam questions, even after classroom discussion of their errors. They do not appear to have "learned from their mistakes". One possible solution is to guide students through a more active process of addressing their mistakes. We will describe a classroom study focused on the assignment of analysing an "Incorrect Answers" page of mistaken statements collected from students' test answers. The students were required to diagnose the statements as part of their homework, and subsequently discuss their answers in class. We will focus on students' difficulties with the process of analysing incorrect statements, specifically in structuring arguments. We will suggest how teachers can direct their practice to foster students' learning in this context.