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The presence of an undetectable fault ui may modify the response of a detectable fault dj to a test set used for fault diagnosis. This may impact the accuracy of fault diagnosis based on the responses of single faults. Many state-of-the-art diagnosis processes are based on the responses of single stuck-at faults even though their goal is to diagnose defects (including multiple defects) that are different from stuck-at faults. Therefore, we study the effects of undetectable single stuck- at faults on the accuracy of fault diagnosis based on the responses of single stuck-at faults. For this purpose, we consider the cases where the response of a double stuck-at fault ui &dj, which consists of an undetectable fault ui and a detectable fault dj, is different from the response of the single fault dj. We show that there are significant, yet manageable, numbers of such faults in benchmark circuits under test sets used for fault diagnosis. In all these cases, a fault diagnosis process based on single stuck-at faults may not identify the locations of dj and ui as candidate defect sites if a defect affects the sites of dj and ui. We conclude that it is important to consider ui&dj during fault diagnosis in order not to preclude the sites of dj and ui as candidate defect sites.