In mission critical systems, especially when software is involved, system-level testing is required to be sure the system/software operates correctly. This is often called Verification & Validation. The two are related, but verification is concerned with testing the system functions against the requirements document (which is essential in a critical system). Each requirement should be testable, and all requirements should be tested. Validation is the process of testing that the system and software behaves as it should when interacting with other systems and components.
Verification & Validation is a long, tedious, and expensive process but is essential for mission-critical systems such as where human life or large investment or one-of-a-kind technology is at risk.