Spectral residuals in the wavelength range near the [O III]λ4363 line, after stellar continuum subtraction. The red and blue Gaussian fits represent [O III]λ4363 and [Fe II]λ4360 emission lines, respectively. The dashed lines illustrate the standard deviation of residual fits in the corresponding windows. The strong-line metallicity and measured [O III] temperature of each stack are reported in the panels. The contamination of the [O III]λ4363 line becomes more relevant with increasing metallicity.