  We investigate whether strong molecular and atomic emission lines at
far-infrared wavelengths can influence the identification and derived
properties of galaxies selected from broad-band, far-infrared or submillimetre
observations. Several of these lines, e.g. [CII]158um, have been found to be
very bright in some high-redshift galaxies, with fluxes of >0.1-1% of the total
far-infrared luminosity, and may be even brighter in certain populations at
high redshifts. At redshifts where these lines fall in instrument pass-bands
they can significantly increase the broad-band flux measurements. We estimate
that the contributions from line emission could boost the apparent broad-band
flux by >20-40% in the Herschel and SCUBA-2 bands. Combined with the steep
source counts in the submillimetre and far-infrared bands, line contamination
has potentially significant consequences for the properties of sources detected
in flux-limited continuum surveys, biasing the derived redshift distributions
and bolometric luminosities. Indeed, it is possible that some z>4 sources found
in 850-um surveys are being identified in part due to line contamination from
strong [CII] emission. These biases may be even stronger for less-luminous and
lower-metallicity populations at high redshifts which are observable with ALMA
and which may have even stronger line-to-continuum ratios.
