Consider this scene of a hollow sphere viewed from the inside:

    difference {
        sphere { 0, 100 }
        sphere { 0, 99 }
        pigment { rgb 1 }
        finish { ambient .4 }
    }

    global_settings {
        radiosity {
            error_bound .1
            always_sample off // default in 3.7
        }
    }

Rendering produces dark splotches at the centers of the pretrace
blocks. Blocks rendered earlier have darker splotches. They also differ
in shape between renders even with +HR (but not with +WT1). Turning
"always_sample" on removes them, as does increasing "count" past 1000.

I wonder if this might be a bug, as 3.6 doesn't produce these artifacts
regardless of "always_sample" and "count".

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