Quantum software tests that pass on broken code expose a silent testing flaw

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A developer identified correctness bugs in three separate quantum computing projects over the past month, all linked to a common testing anti-pattern. In each case, the test assertions checked properties unaffected by the actual bug — such as verifying output magnitudes when the defect was a phase sign error, or truncating string checks before the field containing the wrong value. One Qiskit bug, involving an incorrect sign in a transpiler pass, was confirmed and fixed in version 2.5.1. The author proposes a simple diagnostic question: what code change would actually cause this assertion to fail, and if that cannot be answered quickly, the test is effectively decorative. The bugs were surfaced using a differential fuzzer, with all issue links made publicly available.

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