Since nanoelectronic devices are likely to be defective and error-prone, developing an understanding of circuit reliabilities and critical components will be required. To this end, this paper examines reliability considerations of several sample circuits when implemented in a molecular QCA technology. Probabilistic transfer matrices are used to analyze an XOR, crossover, adder, and an adder using triple modular redundancy. This provides insight in answering how reliable emerging circuit components must be to have a reliable circuit and which of these components are the most critical. As will be shown, component error rates must be at or below 10-4 for an adder to function with 99% reliability and that the straight wire and majority gate are the most critical components to each circuit's reliability. It is also shown that the common assumption made in triple modular redundancy theory that only gates fail is insufficient for QCA.