—Fault injection (FI) has been shown to be an effective approach to assessing the dependability of software systems. To determine the impact of faults injected during FI, a given oracle is needed. Oracles can take a variety of forms, including (i) specifications, (ii) error detection mechanisms and (iii) golden runs. Focusing on golden runs, in this paper we show that there are classes of software which a golden run based approach can not be used to analyse. Specifically, we demonstrate that a golden run based approach can not be used in the analysis of systems which employ a main control loop with an irregular period. Further, we show how a simple model, which has been refined using FI experiments, can be employed as an oracle in the analysis of such a system. Keywords-Fault Tolerance; Software Analysis; Fault Injection; Oracle; Reference Run Model