A CRIRES+ K-band spectrum of MASCARA-1 and the resulting wavelength calibration error. The top panel shows an extracted blaze-corrected spectrum that has been normalized to match the continuum of a telluric model. Here we used the ExoRES pipeline. The bottom panel shows the wavelength calibration error of the same spectrum as estimated using an MCMC. The data shown here are from the lowest S/N spectrum in the MASCARA-1 b data set (S/N ∼ 100), thus illustrating the worst case for this data set. Regions containing fewer telluric lines produce larger calibration errors. We exclude the two spectra between 2.14 and 2.17 μm from further analysis due to large wavelength calibration errors.