An artefact of numerical solution procedures that use a centred scheme for temporal advancement, i.e. one that requires information at three time levels. Starting such a scheme requires two independent initial conditions, one specified and the other calculated from this using a temporal scheme requiring only two time levels. This results in a solution that is actually the sum of two solutions, one related to the actual physics of the problem and the other purely an artefact of the numerical procedure. The numerical solution usually alternates at each time step, resembling a sawtooth wave over time, and can be damped by averaging the solution over two consecutive time steps at suitably chosen intervals.