Now give it a missing case

The machine has all three warning signs. It vibrates near the spinning rod, grows louder when working harder, and feels hotter on one side.

The expert system follows both rules and recommends inspecting the bearing.

But the technician opens the machine and finds a healthy bearing.

The machine had been bolted to an uneven base. That made the motor sit crooked, vibrate, work harder, and grow hot. The rulebook contained no case for that.

Nothing inside the program had broken. It used every rule correctly. The missing possibility was the problem.