Redundant Test Proved Critical When Ninth Code Variant Exposed a Coverage Gap

A software developer testing an order-filter function found that a repeated-status check appeared unnecessary across eight flawed implementations, as other tests already caught those failures. When a ninth implementation was added — one that substituted an identity check with a truthiness test — only the seemingly redundant test detected the bug. The issue stems from Python treating both None and an empty list as falsy, causing the function to incorrectly return all orders when an empty filter is passed. Mutation testing tool mutmut, run on CPython 3.14.6, never automatically generated the critical truthiness-substitution variant, meaning the gap only surfaced when the candidate was added manually. The developer has published all code, mutation diffs, and test results on GitHub, noting the findings are specific to this fixture and should not be generalised to broader mutation testing scenarios.