Architectural mismatch results from implicit and conflicting assumptions that designers of components make about the environments in which these components should operate. While architectural mismatch was extensively studied in monolithic and distributed applications, it has not been applied to service-oriented architectures (SOAs). A major contribution of this paper is the analysis of how architectural mismatch affects SOAs. We study how implicit and conflicting assumptions that designers make about web services and their compositions affect the quality of resulting SOA-based systems. We support our analysis with empirical data that we collected from a large-scale SOA-based project within Accenture and other smaller projects.