“…Such a verification is the main purpose of a posteriori error estimation methods. Several approaches for deriving various a posteriori estimates for elliptic problems for errors measured in global (energy) norms ( [1], [4], [5], [17], [20], [39], [46], [47], [50]), or in terms of various local quantities ( [6], [12], [19], [25], [40], [45]) have been suggested (see also references in the above mentioned works). However, most of the estimates proposed there strongly use the fact that the computed solutions are true finite element (FE) approximations which, in fact, rarely happens in real computations, e.g., due to quadrature rules, forcibly stopped iterative processes, various round-off errors, or even bugs in computer codes.…”