When using numerical computation as a scientific tool, one has to include validation and verification as an internal part of the computing process. One of the elements of that process is assessment of the computational error: that is, the difference between the analytical solution and the numerical solution. Without knowing the analytical solution this seems like an impossible task; however, the theory of a-posteriori error estimators and the practice of error indicators comes to the rescue. The former has been developed mainly for finite elements and is deeply rooted in functional analysis. The latter is easy to apply especially for finite difference methods but is not as firmly based on theory. In the talk we give an overview of theoretical and practical issues as well as present a technical proof of an estimator for mixed finite element methods.