Case one
A test that checked something other than its own name
toqito, a quantum information theory library in Python
Their ruff config turns on five rule sets and bugbear is not one of them. I ran their code with the config disabled and --select B. One real thing came out of the noise, a blind pytest.raises(Exception).
The test is called test_is_ppt_non_hermitian_matrix, so by its name it guards that the function rejects a non-Hermitian matrix. What it caught was an array reshape error. is_ppt takes subsystem dimensions as the square root of the matrix shape, for a 2×2 that gives [[1,1],[1,1]], and partial_transpose dies on the reshape. Hermitian-ness is never checked.
The proof is two probes. A Hermitian 2×2 raises the exact same error. A non-Hermitian 4×4 returns False without complaint. So the test would pass where it has to fail, and would miss the very thing it was written for.
The patch renames the test to what it really checks and narrows pytest.raises(Exception) to ValueError with match, the way the rest of their suite does it. 598 tests green in state_props, their ruff clean. Whether is_ppt should reject a non-Hermitian matrix at all is a separate question, raised in the pull request and left to the maintainer. That one changes library behaviour.
The maintainer reproduced all three points of the proof on his own machine and wrote it out line by line in the review. Merged in 9.4 hours. That review is worth more than the merge.