Or, you may be referring to IDL splotchies appearing at mesh intersections, especially on walls or in figures at the lids and lips. Only solution I know for that is to add low intensity points strategically and correspondingly lower your main light intensity to balance. You want all those bounces to get into the nooks and crannies. Apparently IDL splotchies are a thing of the past with Superfly.

And those 2 posts cover just about every kind of render artifact I've run into. Barring flipped normals and other such extreme cases.

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