In a previous paper, Kuhn [1999] showed that faults in Boolean specifications constitute a hierarchy with respect to detectability, and drew the conclusion that missing condition faults should be hypothesized to generate tests. However this conclusion was premature, since the relationships between missing condition faults and faults in other classes have not been sufficiently analyzed. In this note, we investigate such relationships, aiming to complement the work of Kuhn. As a result, we obtain an extended hierarchy of fault classes and reach a different conclusion.