Are our design for testability features fault secure?

We analyze the risks associated with faults affecting some common design for testability (DFT) features employed within digital products. We will show that some DFT structures may become useless, with consequent dramatic impact on test effectiveness and product quality. We borrow the fault secure property and we will show that it guarantees that no escapes or false acceptance of faulty products may occur because of faults within the DFT structures.