A small number of bipolar planetaries -including M2-9, M1-91, GL 618, and M2-56- exhibit very strong emission lines of low ionization species such as [O I], [N I], [N II], and [S II]. Most previous authors have attempted to analyze the spectra of these objects assuming that they are photoionized by their central stars. Closer examination, however, suggests that a different excitation mechanism may be at work -that of shock heating.