Test cases often reveal missing pieces of knowledge.
Example: Fever of Unknown Origin
Patient is 17-year-old female; persistent fever, headache, lethargy, cardiac symptoms, ...
Diagnoses: Lung cancer (.81), Endocarditis (7).
Correct diagnosis was endocarditis. Physician expert said the diagnosis of lung cancer was consistent with the symptoms; however, lung cancer would never be expected in a patient this young.
Result: new rules added to rule out certain cancers in young patients. Patient age can be used to determine certainty factor of ruling out lung cancer.