Big Pistons
I agree with all you say but in this case the problem was masked by the fact that the system worked perfectly on the ground as there was no ram air to deform the carb heat valve.
Once the aircraft became airborne the ram air going into the intake deformed the valve due to a crack that was hidden from view by the locking wire that held the bolts for the input shaft secure.
No matter how well a pilot did the pre flight checks of the system this defect would not have shown up on the ground, any visual check by a pilot would not have seen the crack as it was hidden from view and the variables of airborne operation hid the defect until a major ice encounter.
The problem was only found in the hangar after the icing incident referred to above when pressure was applied to carb heat valve to simulate airborne operation, as soon as this was done the crack opened up and the defect became apparent.
I have no doubt that the crack quickly propagated between maintenance checks from something so small that detection was very unlikely to the three inch crack right across the width of the valve that caused the problem, it all goes to show that no matter how careful you are aircraft will invent new ways to kill you.