  Strong gravitational lensing is a competitive tool to probe the dark matter
and energy content of the universe. However, significant uncertainties can
arise from the choice of lens model, and in particular the parameterisation of
the line of sight. In this work, we consider the consequences of ignoring the
contribution of foreground perturbers in lens modelling. We derive the explicit
form of the degeneracy between the foreground shear and the ellipticity of a
power law lens, which renders both quantities effectively unmeasurable from
strong lensing observables. Nonetheless, we demonstrate that this degeneracy
does not affect measurements of the Einstein radius. Foreground tidal effects
are also not expected to bias the slope of the potential, and any biases in
this slope should not affect the recovery of the Hubble constant. The
foreground convergence term adds an additional uncertainty to the measurement
of $H_0$, and we show that this uncertainty will be on the order of $1\%$ for
lensing systems located along random lines of sight. There is evidence to
indicate that the probability of strong lensing is higher towards overdense
lines of sight, and this could result in a small systematic bias towards
overestimations of $H_0$.
