  We derive a fundamental conservation law of operator current for master
equations describing reduced quantum systems. If this law is broken, the
temporal integral of the current operator of an arbitrary system observable
does not yield in general the change of that observable in the evolution. We
study Lindblad-type master equations as examples and prove that the application
of the secular approximation during their derivation results in a violation of
the conservation law. We show that generally any violation of the law leads to
artificial corrections to the complete quantum dynamics, thus questioning the
accuracy of the particular master equation.
