A fundamental reason for investing in product families is to minimize the application engineering costs. Several organizations that employ product families, however, are becoming increasingly aware of the fact that, despite the efforts in domain engineering, deriving individual products from their shared software assets is a time and effort consuming activity. In this paper we present a collection of product derivation problems that we identified during a case study at two large and mature industrial organizations. These problems are attributed to the lack of methodological support for application engineering, and to underlying causes of complexity and implicit properties. For each problem, we provide a description and an example, while for each cause we present a description, consequences, solutions and research issues. The discussions in this paper are relevant outside the context of the two companies, as the challenges they face do or eventually will arise in, for example, comparabl...