In this study, preservice preschool teachers' pedagogical content knowledge about geometric shapes was examined in the context of children's mistakes. A total of 84 preservice teachers, who were studying in preschool teacher education program at a state university in Turkey, participated in this study. Of the preservice teachers, 34 were sophomore (second-year) and 50 were junior (third-year) undergraduate students. The illustrative case study method, a qualitative research method, was used. Seven different vignettes were developed by researchers and used as data collection tools. Each vignette had a typical geometrical mistake common for preschool-age children. The data obtained from open-ended vignettes were analyzed through the summative content analysis technique. In conclusion, it was observed that the preservice teachers succeeded in partially identifying children's mistakes in many vignettes. However, the preservice teachers failed to propose solutions for eliminating children's mistakes.