Our red-team run plants small bugs in a product's code and counts how many the tests notice. On the patent fee planner's usage counter, 7 of 8 were caught. The one that was not flipped the default of writeOptOut so that calling it with no choice would turn tracking off for the visitor instead of leaving it on.
It is a small slip, but a privacy control that silently changes state is the kind a test should pin. We added the test, re-ran the planted bug, and it now fails as it should. The same counter file is copied into a couple of hundred of our tools, so the check is worth repeating where it is missing.