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In order to remain tractable, mhany reliability models do not include the states and transitions necessary to represent fault/error-handling details. Instead, the effectiveness of fault/ error-handling mechanisms is represented by the use of instantaneous coverage probabilities. This paper investigates the effect of the error introduced by the assumption of instantaneous coverage probabilities on the predictions of the reliability model, and it shows that the reliability estimates thus obtained are lower bounds on the reliability estimates of the composite model with embedded fault/error-handling states and transitions. The paper also discusses the choice of the calculation method for the instantaneous coverage probabilities and defines a near-coincident-fault coverage model that yields conservative instantaneous coverage probabilities.