Testing C-elements is not elementary

We examine stuck-at faults in several gate circuits realizing the C-element. We exhibit circuits with the following phenomena: (a) 50% of single faults do not cause the circuit to halt. (b) Some faults are not detectable by logic tests. (c) A test of length seven is required to detect all detectable single faults. (d) A fault may result in an oscillation. (e) A fault may destroy the speed-independence of a circuit. We also analyze static and dynamic CMOS implementations of the C-element.