  Diffuse Ionized Gas (DIG) is prevalent in star-forming galaxies. Using a
sample of 365 nearly face-on star-forming galaxies observed by MaNGA, we
demonstrate how DIG in star-forming galaxies impacts the measurements of
emission line ratios, hence the interpretation of diagnostic diagrams and
gas-phase metallicity measurements. At fixed metallicity, DIG-dominated low
H\alpha\ surface brightness regions display enhanced [SII]/H\alpha,
[NII]/H\alpha, [OII]/H\beta, and [OI]/H\alpha. The gradients in these line
ratios are determined by metallicity gradients and H\alpha\ surface brightness.
In line ratio diagnostic diagrams, contamination by DIG moves HII regions
towards composite or LI(N)ER-like regions. A harder ionizing spectrum is needed
to explain DIG line ratios. Leaky HII region models can only shift line ratios
slightly relative to HII region models, and thus fail to explain the
composite/LI(N)ER line ratios displayed by DIG. Our result favors ionization by
evolved stars as a major ionization source for DIG with LI(N)ER-like emission.
  DIG can significantly bias the measurement of gas metallicity and metallicity
gradients derived using strong-line methods. Metallicities derived using N2O2
are optimal because they exhibit the smallest bias and error. Using O3N2, R23,
N2=[NII]/H\alpha, and N2S2H\alpha\ (Dopita et al. 2016) to derive metallicities
introduces bias in the derived metallicity gradients as large as the gradient
itself. The strong-line method of Blanc et al. (2015; IZI hereafter) cannot be
applied to DIG to get an accurate metallicity because it currently contains
only HII region models which fail to describe the DIG.
