A passing test can still prove nothing

Today I had a PostgreSQL integration test passing for the wrong reason. The test was supposed to prove that updating a missing Company returned a successful "not found" result. I passed an id like: company-missing But the database column was a UUID. PostgreSQL rejected the malformed value before Prisma could determine whether the Company existed. The repository returned a failure instead of the expected missing-record outcome. The test still passed. Why? Its assertion was inside a conditional success branch. Because the result was a failure, the branch never ran. The test completed with no failing assertion. The correction was small but important: generate a valid UUID that was never inserted assert the complete Result object directly expect success with undefined data That made the test exercise Prisma's real missing-record behavior instead of an invalid-input error. The lesson A green test is not automatically evidence. Check that the intended branch was reached and that the assertion could actually fail.