On one of my own projects, every automated check came back green on a version of the main file that could not start. Fifty-three checks, all passing, on code that was broken.
Nothing went wrong with the checks. They ran exactly as written. Not one of them ever opened that particular file, and the check that runs before work is saved was only looking at the documentation. Both were working. Neither could see this.
I've since collected the same shape elsewhere. A setting meant to point a job at a test database, which looked right and was never read by anything, so the job used the live one. A one-click fix for three compliance items, built and tested and clean, where those items turn out to be the kind you can only promise you've done rather than prove. It would have ticked the boxes and nobody would have done the thing behind them.
What all of them have in common took me a while to see. The check wasn't broken. It answered its own question correctly. Its question and mine simply had the same name.
"Did the output change" is not "did my change alter the output". "Does this word appear on the page" is not "does the code use this word". "How many of these match my search" is not "how many of these exist". Each pair sounds like one question right up to the moment they disagree, and nothing in a green light tells you which one you just asked.
Notice that none of these are bugs in the ordinary sense. Nothing regressed. Each had never worked the way its label said, and a check that asks whether today matches yesterday will agree with you about that forever.
It matters more now because work arrives faster and arrives looking finished. Formatted, plausible, tests green, with a confident summary at the end that is the least reliable part of it. Speed hasn't made the work worse. It's made the surface smoother, and the surface is what we inspect.
So I've started asking something duller than "did it pass". What would have to be true for this to fail, and can this check even see that? If I can't answer, the green light is decoration.
Not what does this protect. What is it structurally incapable of noticing.