In this paper, we prove that, under precise spectral assumptions, some finite difference approximations of scalar leftgoing transport equations on the positive half-line with numerical boundary conditions are
ℓ
1
\ell ^1
-stable but
ℓ
q
\ell ^q
-unstable for any
q
>
1
q>1
. The proof relies on the accurate description of the Green’s function for a particular family of finite rank perturbations of Toeplitz operators whose essential spectrum belongs to the closed unit disk and with a simple eigenvalue of modulus
1
1
embedded into the essential spectrum.