The great difficulties that are encountered when reliability requirements for critical software have to be validated motivate an approach that facilitates testing for exceptional conditions that the software is expected to handle. It is shown that in several published studies, failures in previously tested critical programs occurred when rare events were encountered in the execution. Statement of requirements in the form of condition tables assures that all combinations of rare events that must be handled by the program are clearly recognized during development, and the condition tables can be translated directly into test cases that validate the conformance with these requirements.