  In this paper we show how to find the exact error (not just an estimate of
the error) of a conforming mixed approximation by using the functional type a
posteriori error estimates in the spirit of Repin. The error is measured in a
mixed norm which takes into account both the primal and dual variables. We
derive this result for elliptic partial differential equations of a certain
class. We first derive a special version of our main result by using a
simplified reaction-diffusion problem to demonstrate the strong connection to
the classical functional a posteriori error estimates of Repin. After this we
derive the main result in an abstract setting. Our main result states that in
order to obtain the exact global error value of a conforming mixed
approximation one only needs the problem data and the mixed approximation of
the exact solution. There is no need for calculating any auxiliary data. The
calculation of the exact error consists of simply calculating two (usually
integral) quantities where all the quantities are known after the approximate
solution has been obtained by any conforming method. We also show some
numerical computations to confirm the results.
