Contribution of various beam-window-function-related errors and uncertainties to the C_ℓ relative error. In each panel, the grey histogram shows the relative statistical error on the Planck CMB TT binned power spectrum (for a bin width Δℓ = 30) divided by 10, while the vertical grey dashes delineate the range ℓ< 1800 that is most informative for base ΛCDM. Top: estimation of the error made by ignoring the sub-pixel effects for a fiducial C_ℓ including the CMB and CIB contributions. Middle: error due to the sky mask, for the Galactic masks used in the TT analysis. Bottom: current beam window function error model, shown at 1σ (solid lines) and 10σ (dotted lines).