Simulation is a popular approach for empirically evaluating the performance of algorithms and applications in the parallel computing domain. Most published works present results without quantifying simulation error. In this paper, the authors investigate accuracy issues when simulating the execution of parallel applications. This is a broad question, and they focus on a relevant case study: the evaluation of scheduling algorithms for executing mixed-parallel applications on clusters. Most such scheduling algorithms have been evaluated in simulation only. They compare simulations to real-world experiments in a view to identify which features of a simulator are most critical for simulation accuracy.