Observed spectra (black) of SN 1999em (left) and 2007aa (right) at 165 and 105 days from the explosion. The vertical dashed lines show the rest-frame position of the strongest lines. The simulated spectrum of SN 2007aa, where a fake Na I D narrow line was added at λ = 5893 Å (EW= 1.1 Å), is presented on top of the observed spectrum (cyan). The peak of the Na I D seems redshifted in SN 1999em, compared to Hα, which is at the rest-position. A comparable shift can be obtained when a narrow line is located on the top of the broad SN P-Cygni line of SN 2007aa.