The concept of pedagogical content knowledge has been explored in the context of several disciplines, such as mathematics, medicine and chemistry. In this paper the concept is explored and applied to the subject matter of computer science, in particular to the subdomain of building UML class diagrams. It is argued that the identification and analysis of problems that students experience with important concepts should be at the heart of pedagogical content knowledge in this subdomain, as well as the description of pedagogically rich exercises for tackling those problems. For two examples, relevant pedagogical content knowledge is identified and represented in the form of educational cases.