A challenge in software design is to find solutions that balance and optimize the quality attributes of the application. In this paper we present a case study of an application and the results of a design decision made on weak assumptions. The application has been assessed with respect to performance and maintainability. We present and evaluate an alternative design of a critical system component. Based on interviews with the involved designers we establish the design rationale. By analyzing the evaluation data of the two alternatives and the design rationale, we conclude that the design decision was based on a general assumption that an adaptable component design should increase the maintainability of the application. This case study is clearly a counter example to that assumption, and we therefore reject it as a myth. This study shows, however, that the myth is indeed responsible for the major performance problem in the application.