  We compare the most successful and widely used map of Galactic dust
extinction, provided by Schlegel, Finkbeiner & Davis (1998; hereafter SFD), to
the galaxy number counts in the Sloan Digital Sky Survey (SDSS)
photometric/spectroscopic DR4 sample. We divide the SDSS survey area into 69
disjoint subregions according to the dust extinction provided by SFD and
compare the surface number density of galaxies in each subregion. As expected,
the galaxy surface number density decreases with increasing extinction but only
for SFD extinction values above about 0.1 to 0.2 magnitudes (depending on the
band). At lower values of the SFD extinction, we find that the sky surface
density of galaxies increases with increasing extinction, precisely the
opposite of the effect expected from Galactic dust. We suggest that the far
infrared (FIR) brightness of the sky in regions of true low dust extinction is
significantly ``contaminated'' by the FIR emission from background galaxies. We
show that such an explanation is both qualitatively and quantitatively
consistent with the available data. Based on this interpretation we conclude
that systematic errors in the SFD extinction map due to extragalactic FIR
emission are quite small, of order hundredths of a magnitude, but nevertheless
statistically detectable. (Abridged)
