Brushes Near Curved Surfaces Light Improperly

‘Smoothing’ between the lighting of two brushes (most commonly performed on a section of curved wall) causes dark patches of varying sizes to appear on those brushes and brushes nearby. Disabling smoothing causes these anomalies to go away, but of course without smoothing the resulting lights are often unrealistic:

Back wall and curve smoothed together.

Back wall separated from curve smoothing group.

Smoothing disabled globally with -smooth 0.