Computer simulations are not replicas of reality.  For example, Boeing Co. Engineers used simulation to design a fuse pin that held the engines to the wing for its 747 cargo plane.  After El Al Israel Airlines had a crash in 1992, where the plane killed over 40 people in the Netherlands, engineers reviewed their simulation.  They found that the simulation had missed several weak points in the design of the fuse pin.  The fuse pin had, in fact broken, causing the crash.