  The paper is concerned with functional type a posteriori estimates for the
initial boundary value problem for a parabolic partial differential equation
with an obstacle. We deduce a guaranteed and computable bound of the distance
between the exact minimizer and any function from the admissible (energy) class
of functions. Applications to the analysis of modeling errors caused by data
implification are discussed. An important case of time incremental
approximations is specially studied. Numerical examples presented in the last
section show how the estimates work in practice.
