[Steuard]

Right, chaotic behavior is a valid issue. But in any realistic situation, you'll have lots of effects that inevitably get left out of simulations that contribute larger perturbations/errors than this rounding error would. (Heck, I suspect that even the thermal motion of the individual atoms in an object would have a larger effect.)

So no simulation will ever get exactly the "right" answer at that level. For practical purposes, all we care about is getting an answer that's within the ensemble of reasonable outcomes for initial conditions like ours.