Comment by praptak

Energy preservation doesn't work by default in physics simulations unless you specifically code for it (e.g. by periodic explicit adjustment).

It's not only the rounding error, it's also quantisation of time and other minor errors coming from the mathematical model itself.

If the error is on the damping side, you get the real life effect of motion eventually stopping due to energy dissipation. If it's on the acceleration side you get a runaway simulation.