On a common error in open and short circuit reliability computation

In reliability computations for complex systems subject to two kinds of failure, analysts sometimes underestimate system reliability by using a formula that actually applies only to certain simpler structures. This error occurs several times, for example, in an otherwise instructive paper by B.W. Jenney and D.J. Sherwin (ibid., vol.R-35, p.532-8, Dec. 1986). The purpose of this work is to describe the error, illustrate how it arises in practice, and offer a correction