  The stability bounds and error estimates for a compact higher order
Numerov-Crank-Nicolson scheme on non-uniform space meshes for the 1D
time-dependent Schr\"odinger equation have been recently derived. This analysis
has been done in $L^2$ and $H^1$ mesh norms and used the non-standard
"converse" condition $h_\omega\leq c_0\tau$, where $h_\omega$ is the mean space
step, $\tau$ is the time step and $c_0>0$. Now we prove that such condition is
necessary for some families of non-uniform meshes and any space norm. Also
numerical results show unacceptably wrong behavior of numerical solutions
(their dramatic mass non-conservation) when this condition is violated.
