  Over twenty-five years ago, two analyses of the pion-nucleon ($\pi N$) data
at low energy (i.e., for pion laboratory kinetic energy $T \leq 100$ MeV)
reported on the departure of the extracted scattering amplitudes, corresponding
to the two elastic-scattering reactions $\pi^\pm p \to \pi^\pm p$ and to the
$\pi^- p$ charge-exchange reaction $\pi^- p \to \pi^0 n$, from the triangle
identity, which these amplitudes fulfil if the isospin invariance holds in the
hadronic part of the $\pi N$ interaction. This discrepancy indicates that
\emph{at least one} of the following assumptions is not valid: first, that the
absolute normalisation of the bulk of the low-energy $\pi N$ datasets is
correct; second, that any residual contributions to the corrections, which aim
at the removal of the effects of electromagnetic (EM) origin from the
measurements, are not significant; and third, that the isospin invariance holds
in the hadronic part of the $\pi N$ interaction. In view of the incompatibility
of the results of the various schemes of removal of the so-called trivial EM
effects at the $\pi N$ threshold ($T = 0$ MeV), the likelihood of residual
effects of EM origin in the extracted $\pi N$ scattering amplitudes (from the
data in the scattering region, i.e., above the $\pi N$ threshold) must be
reassessed. This work emphasises the importance of the development of a unified
scheme for the determination of reliable EM corrections, \emph{applicable at
the $\pi N$ threshold and in the scattering region}, in providing a resolution
to the established discrepancy at low energy.
