  Next-generation spectroscopic surveys will map the large-scale structure of
the observable universe, using emission line galaxies as tracers. While each
survey will map the sky with a specific emission line, interloping emission
lines can masquerade as the survey's intended emission line at different
redshifts. Interloping lines from galaxies that are not removed can contaminate
the power spectrum measurement, mixing correlations from various redshifts and
diluting the true signal. We assess the potential for power spectrum
contamination, finding that an interloper fraction worse than 0.2% could bias
power spectrum measurements for future surveys by more than 10% of statistical
errors, while also biasing power spectrum inferences. We also construct a
formalism for predicting cosmological parameter bias, demonstrating that a
0.15%-0.3% interloper fraction could bias the growth rate by more than 10% of
the error, which can affect constraints on gravity upcoming surveys. We use the
COSMOS Mock Catalog (CMC), with the emission lines re-scaled to better
reproduce recent data, to predict potential interloper fractions for the Prime
Focus Spectrograph (PFS) and the Wide-Field InfraRed Survey Telescope (WFIRST).
We find that secondary line identification, or confirming galaxy redshifts by
finding correlated emission lines, can remove interlopers for PFS. For WFIRST,
we use the CMC to predict that the 0.2% target can be reached for the WFIRST
H$\alpha$ survey, but sensitive optical and near-infrared photometry will be
required. For the WFIRST [OIII] survey, the predicted interloper fractions
reach several percent and their effects will have to be estimated and removed
statistically (e.g. with deep training samples). (Abridged)
