I explore the effects of observational errors on nebular chemical abundances using a sample of 179 optical spectra of 42 planetary nebulae (PNe) observed by different authors. The spectra are analysed in a homogeneous way to derive physical conditions and ionic and total abundances. The effects of recombination on the [OII] and [NII] emission lines are estimated by including the effective recombination coefficients in the statistical equilibrium equations that are solved for O^+^ and N^+^. The results are shown to be significantly different than those derived using previous approaches. The O^+^ abundances derived with the blue and red lines of [OII] differ by up to a factor of 6, indicating that the relative intensities of lines widely separated in wavelength can be highly uncertain. In fact, the HeII lines in the range 4000-6800{AA} imply that most of the spectra are bluer than expected. Scores are assigned to the spectra using different criteria and the spectrum with the highest score for each PN is taken as the reference spectrum. The differences between the abundances derived with the reference spectrum and those derived with the other spectra available for each object are used to estimate the 1{sigma} observational uncertainties in the final abundances: 0.11dex for O/H and Ar/H, 0.14dex for N/H, Ne/H, and Cl/H, and 0.16dex for S/H.