Traditionally, in fault injection-based robustness evaluation of software (specifically for Operating Systems - OS's), faults or errors are injected at specific code locations. This paper studies the sensitivity and accuracy of the robustness evaluation results arising from varying the timing of injecting the faults into the OS. A strategy to guide the triggering of fault injection is proposed, based on the observation that the operational usage profile of a driver shows a high degree of regularity in the calls being made. The concept of call blocks (i.e., a distinct sequence of calls made to the driver) can be used to guide injections into different system states, corresponding to the driver operations carried out. A real-world case study compares the effectiveness of the proposed strategy to traditional location-based approaches, demonstrating that significant and useful insights can be gained by modulating the injection instants. © 2007 IEEE.