  Roman Space Telescope will survey about 17 million emission-line galaxies
over a range of redshifts. Its main targets are H$\alpha$ emission-line
galaxies at low redshifts and [O III] emission-line galaxies at high redshifts.
The Roman Space Telescope will estimate the redshift these galaxies with single
line identification. This suggests that other emission-line galaxies may be
misidentified as the main targets. In particular, it is hard to distinguish
between the H$\beta$ and [O III] lines as the two lines are close in wavelength
and hence the photometric information may not be sufficient to separate them
reliably. Misidentifying H$\beta$ emitter as [O III] emitter will cause a shift
in the inferred radial position of the galaxy by approximately 90 Mpc/h. This
length scale is similar to the Baryon Acoustic Oscillation (BAO) scale and
could shift and broaden the BAO peak, possibly introduce errors in determining
the BAO peak position. We qualitatively describe the effect of this new
systematic and further quantify it with a lightcone simulation with
emission-line galaxies.
