The calculation of the total surface area of prisms forms part of the Mathematics curriculum in South Africa. This paper explores Grade 8 students' errors when learning about the surface area of prisms. The study was guided by the interpretive paradigm and adopted a qualitative approach in a form of a case study design. Data from diagnostic test's responses were analysed using Newman's theory of error analysis. The sample comprised 18 purposively selected Grade 8 students. The study revealed that students committed these errors: failure to differentiate 2-Dimensionals from 3-Dimensionals; calculated the Total Surface Area as the perimeter of polygons; used volume of prisms formula for Total Surface Area; and misunderstanding of mathematical terminology related to surface area of prisms. The study recommends the use of students' errors as tools to guide in designing the intervention activities to remedy misconceptions that students hold when learning about the surface area of prisms.