A discretization of a convection-diffusion-reaction problem should provide a numerical solution that is in some sense a good approximation of the analytic solution. In numerical analysis, the quality of approximation is usually measured in norms of function spaces. However, from the practical point of view, it is often of utmost importance that the numerical solution is physically consistent, i.e., that it possesses some basic physical properties that are valid in the same form for the solution of the continuous problem.