I agree, it's probably not the cause of the artifacts you see.

However, it will cause corners where two flat planes meet to blow up. Any slight numerical variations in angles in the last bounce could result in a localized difference of near 1.0 between two close points in the GI calculation.

When done with DV = .8, the contribution of the last bounce is only 26% versus 100% when 6 bounces are used. Ideally, you'd use more bounces than that, getting down to 1-2% variation in the last bounce.

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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)