Scenario based requirements specifications are the industry norm for defining communicating systems. These scenarios are often captured in the form of UML/MSC sequence diagrams. Errors are often introduced at this stage of the development process, which are costly to resolve if they are not detected early. This paper is concerned with the automatic detection and resolution of semantic errors that can occur in such scenarios.The paper discusses a semantic interpretation of scenario-based requirements and various types of defects (or pathologies) that can be detected. The paper defines the semantics and defects within a partial order theoretic framework. We introduce a UML 2.0 profile that captures various domain specific communication semantics, which can be used to determine the relevance of detected pathologies when different underlying implementation assumptions are made. The paper also discusses how to automatically resolve pathologies by using this profile to adapt the communication architecture in the requirements model.