Left panel: Example of redshift contaminants due to [S III]λ9531 at 1752.7 nm mis-identified as Hα, yielding z_meas = 1.67 instead of z_true = 0.836. Right panel: Median (in blue) and mean (in red) stack of 200 spectra (in grey) of galaxies where the [S III]λ9531 line was misinterpreted as Hα. Vertical dotted lines indicate the ‘true’ lines, while the coloured bands highlight the expected rest-frame positions of key optical emission lines at the wrong redshift of the stacked spectrum (Hβ, [O III], Hα, and [N II]).