  A fundamental assumption adopted in nearly every extragalactic emission-line
study is that the attenuation of different emission lines can be described by a
single attenuation curve. Here we show this assumption fails in many cases with
important implications for derived results. We developed a new method to
measure the differential nebular attenuation among three kinds of transitions:
the Balmer lines of hydrogen, high-ionization transitions, and low-ionization
transitions. This method bins the observed data in a multidimensional space
spanned by attenuation-insensitive line ratios. Within each small bin, the
variations in line ratios are mainly driven by the variations in the nebular
attenuation. This allows us to measure the nebular attenuation using both
forbidden lines and Balmer lines. We applied this method to a sample of 2.4
million star-forming spaxels from SDSS-IV MaNGA. We found that the attenuation
of high ionization lines and Balmer lines can be well described by a single
Fitzpatrick (1999) extinction curve with $R_V=3.1$. However, no single
attenuation curve can simultaneously account for all three transitions. This
strongly suggests that different lines have different effective attenuations,
likely because spectroscopy at kiloparsec resolutions mixes multiple regions
with different intrinsic line ratios and different levels of attenuation. As a
result, the assumption that different lines follow the same attenuation curve
breaks down. Using a single attenuation curve determined by Balmer lines to
correct attenuation-sensitive forbidden line ratios could bias the nebular
parameters derived by 0.06--0.25 dex at $A_V = 1$, depending on the details of
the dust attenuation model. Observations of a statistically large sample of H
II regions with high spatial resolutions and large spectral coverage are vital
for improved modeling and deriving accurate corrections for this effect.
