  (Original) Recent high-resolution spectra of the Type Ia SN 2006X have
revealed the presence of time-variable and blueshifted Na I D features,
interpreted by Patat et al. as originating in circumstellar material within the
progenitor system. The variation seen in SN 2006X induces relatively large
changes in the total Na I D equivalent width ($\Delta\rm{EW}\approx 0.5
\unicode{x212B}$ in just over two weeks), that would be detectable at lower
resolutions. We have used a large data set comprising 2400 low-resolution
spectra of 450 Type Ia supernovae (SNe Ia) obtained by the CfA Supernova
Program to search for variable Na I D features. Out of the 31 SNe Ia (including
SN 2006X) in which we could have detected similar EW variations, only one other
(SN 1999cl) shows variable Na I D features, with an even larger change over a
similar ~10-day timescale ($\Delta\rm{EW} = 1.66 \pm 0.21 \unicode{x212B}$).
Interestingly, both SN 1999cl and SN 2006X are the two most highly-reddened
objects in our sample, raising the possibility that the variability is
connected to dusty environments.
  (Erratum) The large variation in the Na I D equivalent width observed in SN
1999cl results in fact from a measurement error. Our new measurements show that
the EW variation is significantly lower, at $0.43 \pm 0.14 \unicode{x212B}$.
While the EW variation remains statistically significant (3.1$\sigma$ different
from zero), it is now below the detection threshold of 0.5 $\unicode{x212B}$
derived from the Monte Carlo simulations published in the original paper. As a
result, SN 1999cl should no longer be considered as an object displaying
variable Na I D lines in our study. The fraction of SNe Ia in our sample
displaying Na I D lines thus goes from $\sim$6% (2/31) in the original study to
$\sim$3% (1/31) in the revised analysis, SN 2006X being the only SN Ia in our
sample with variable Na I D lines.
