An underlying assumption of component-based software engineering for interactive systems is that the overall usability of a new assembled device mainly depends on the usability of its individual components. This paper challenges this assumption by presenting findings from a series of lab experiments in which 48 subjects operated several consumer devices. The experiments focussed on the effect inconsistency may have on the usability of individual components. The results indicate that inconsistency could cause components, in the same or in higher layers, to activate an inappropriate mental model for other components. Furthermore, the application domain also seems to have an effect on the subjects understanding of the functionality a component provides.