The calculation of the pressure distribution, overall lift and moment coefficients on a two-dimensional aerofoil at low speeds, including boundary layer effects, is nowadays a fairly standard numerical technique. Experience has shown that as long as the flow remains attached, theory predicts experimental results extremely well for a ‘clean’ aerofoil. The purpose of this note is to point out that the application of the same theory to aerofoils with conventional trailing edge control surfaces leads to results which differ significantly from those measured in experiments.