  We perform forecasts for how baryon acoustic oscillation (BAO) scale and
redshift-space distortion (RSD) measurements from future spectroscopic emission
line galaxy (ELG) surveys such as Euclid are degraded in the presence of
spectral line misidentification. Using analytic calculations verified with mock
galaxy catalogs from log-normal simulations we find that constraints are
degraded in two ways, even when the interloper power spectrum is modeled
correctly in the likelihood. Firstly, there is a loss of signal-to-noise ratio
for the power spectrum of the target galaxies, which propagates to all
cosmological constraints and increases with contamination fraction, $f_c$.
Secondly, degeneracies can open up between $f_c$ and cosmological parameters.
In our calculations this typically increases BAO scale uncertainties at the
10-20% level when marginalizing over parameters determining the broadband power
spectrum shape. External constraints on $f_c$, or parameters determining the
shape of the power spectrum, for example from cosmic microwave background (CMB)
measurements, can remove this effect. There is a near-perfect degeneracy
between $f_c$ and the power spectrum amplitude for low $f_c$ values, where
$f_c$ is not well determined from the contaminated sample alone. This has the
potential to strongly degrade RSD constraints. The degeneracy can be broken
with an external constraint on $f_c$, for example from cross-correlation with a
separate galaxy sample containing the misidentified line, or deeper
sub-surveys.
