We show that a single stuck-at fault in a non-redundant delay-insensitive circuit results in a transition either not taking place or firing prematurely, or both, during an execution of the circuit. A transition not taking place can be tested easily, as this always prevents a transition on a primary output from taking place. A premature firing can also be tested but the addition of testing points may be required to enforce the premature firing and to propagate the transition to a primary output. Hence all single stuck-at faults are testable. All test sequences can be generated from the high-level specification of the circuit. The circuits are hazard-free in normal operation and during the tests.