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When using output response compaction it is necessary to address the fact that circuits may produce unknown output values. Methods to address this issue ensure that the output response compactor would produce a unique fault-free signature that can be used for fault detection. This paper considers the unknown output values that are produced by faulty circuits. If a faulty circuit produces an unknown value when the output value of the fault-free circuit is known, the unknown faulty output value may affect the computation of a signature. The faulty signature may not be unique, and it may not always be different from a fault-free signature. The ability to verify that the fault will be detected based on its signature is thus lost. Without limiting the discussion to a particular output response compactor, the paper studies the prevalence of such faults in benchmark circuits, the prevalence of unknown faulty output values for which the corresponding fault-free values are known, and the effects of addressing this issue.