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In trace-driven simulation, traces generated for one set of machine characteristics are used to simulate a machine with different characteristics. However, the execution path of a multiprocessor workload may depend on the ordering of events on different processors, which in turn depends on machine characteristics such as memory system timings. Trace-driven simulations of multiprocessor workloads are inaccurate unless the timing-dependencies are eliminated from the traces. We measure such inaccuracies by comparing trace-driven simulations to direct simulations of the same workloads. The results were identical only for workloads whose timing dependencies were eliminated from the traces. The remaining workloads used either first-come first-served scheduling or non-deterministic algorithms; these characteristics resulted in timing-dependencies that could not be eliminated from the traces. Workloads which used task-queue scheduling had particularly large discrepancies because task-queue operations, unlike other synchronization operations, were not abstracted. Two types of simulation results had especially large discrepancies: those related to synchronization latency and those derived from relatively small numbers of events. Studies that rely on such results should use timing- independent traces or direct simulation.