Programs must be considered faulty even though we could not devise test cases that expose faults or failures. It is a typically area the examples. The Therac versions did work for a long time. So the situation has never created a test case, which caused it to fail. So, the fault was not uncovered till almost 1987. It is equally true Ariane. The fault that the acceleration above a certain limit causes a overflow. This particular aspect was not considered in the design and so it has led to a failure. This fault was uncovered only when Ariane 5 failed.