  We reanalyze the problem of a 1D Dirac single particle colliding with the
electrostatic potential step of height $V_{0}$ with a positive incoming energy
that tends to the limit point of the so-called Klein energy zone, i.e.,
$E\rightarrow V_{0}-\mathrm{m}c^{2}$, for a given $V_{0}$. In such a case, the
particle is actually colliding with an impenetrable barrier. In fact,
$V_{0}\rightarrow E+\mathrm{m}c^{2}$, for a given relativistic energy
$E\,(<V_{0})$, is the maximum value that the height of the step can reach and
that ensures the perfect impenetrability of the barrier. Nevertheless, we note
that, unlike the nonrelativistic case, the entire eigensolution does not
completely vanish, either at the barrier or in the region under the step, but
its upper component does satisfy the Dirichlet boundary condition at the
barrier. More importantly, by calculating the mean value of the force exerted
by the impenetrable wall on the particle in this eigenstate and taking its
nonrelativistic limit, we recover the required result. We use two different
approaches to obtain the latter two results. In one of these approaches, the
corresponding force on the particle is a type of boundary quantum force.
Throughout the article, various issues related to the Klein energy zone, the
transmitted solutions to this problem, and impenetrable barriers related to
boundary conditions are also discussed. In particular, if the negative-energy
transmitted solution is used, the lower component of the scattering solution
satisfies the Dirichlet boundary condition at the barrier, but the mean value
of the external force when $V_{0}\rightarrow E+\mathrm{m}c^{2}$ does not seem
to be compatible with the existence of the impenetrable barrier.
