  The planetary nebula luminosity function (PNLF) has been used as an
extragalactic distance indicator since the 1980's, but there are still unsolved
problems associated with its use. One of the most serious involves PNLF
distances beyond ~ 10 Mpc, which tend to be slightly smaller than those of
other methods. We consider the implications of previous spectroscopic
investigations that found that several of the brightest planetary nebula (PN)
candidates in M74 are actually compact supernova remnants (SNRs). Using
narrow-band imaging data from the KPNO 4-m telescope, we measure the [O III]
$\lambda$5007 and H$\alpha$ fluxes of all the known SNRs in M31 and M33, and
test whether those objects could be misidentified as bright PNe at distances
beyond ~ 10 Mpc. Our results suggest that compact SNRs are not an important
source of contamination in photometric surveys for extragalactic PNe.
