Non-functional requirements are easy to overlook. This is most likely because stakeholders often assume that these requirements are given and go without saying. Missing requirements are harder to spot during requirements evaluation than poorly specified requirements and are usually detected further down the line when the system is in the testing phase or has already been deployed to thousands of users. They become even more expensive to fix if they are architecturally significant. For example, It is often difficult to include performance, safety and security features to an existing architecture - Donald Firesmith, Software Engineering Institute, U.S.A.