  Proposed cosmological surveys will make use of photometric redshifts of
galaxies that are significantly fainter than any complete spectroscopic
redshift surveys that exist to train the photo-z methods. We investigate the
photo-z biases that result from known differences between the faint and bright
populations: a rise in AGN activity toward higher redshift, and a metallicity
difference between intrinsically luminous and faint early-type galaxies. We
find that even very small mismatches between the mean photometric target and
the training set can induce photo-z biases large enough to corrupt derived
cosmological parameters significantly. A metallicity shift of ~0.003dex in an
old population, or contamination of any galaxy spectrum with ~0.2% AGN flux, is
sufficient to induce a 10^-3 bias in photo-z. These results highlight the
danger in extrapolating the behavior of bright galaxies to a fainter
population, and the desirability of a spectroscopic training set that spans all
of the characteristics of the photo-z targets, i.e. extending to the 25th mag
or fainter galaxies that will be used in future surveys.
