The artifacts are harder to see when the area of difficulty also gets significant direct light. In such cases, the indirect light is only a fraction of the light, so the degree of artifacting is also reduced to a fraction.

As seen in Joe Public's shelf render, the shadowed area of the shelf reveleals the artifact. Note that the shelf artifact is huge due to using very broad IC samples.

Here in my render, the bottom of the prop shows the artifact very badly because this side is in shadow - the sun doesn't reach - so all of this lighting back here is indirect.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)