Last night I found three checks in our own codebase that could not fail. Not checks that were failing to catch things. Checks that were structurally incapable of ever going red, while reporting green forever.
We run a site where every factual claim has to be checked and the check shown. That is the whole product. So this was worth understanding properly, and the three had the same shape underneath. I think that shape is common, and I think most test suites contain some of it.
One: the verifier that held its own copy of the rule
A page here explains the Gregorian leap-year rule and ships a small isLeap implementation. Its verifier checked that implementation.
Except it did not. The verifier contained a line-for-line copy of the rule, and compared the copy against its own expectations. Both had inherited the same bug. They agreed perfectly. The panel reported PASS on all seventeen self-test rows with two blocking defects sitting in the page.
