I threw a "real world" scene at it. Thinking about how else I could exploit volume caustics in lack of refractive materials, I figure it would be cool to build a solar thermal power plant model.
Modelled in Stud.io and Cinema 4D.
677761 Triangles, most of them occluded due to the bricks being stuck in each other.
8700 SPP in 34 h
The scene is contained in a box with a scattering medium, obviously. Fireflys due to SDS paths? I'm not sure, but I suppose there can be paths from camera to mirror to volume scattering to mirror to sun. The sun spans a small solid angle, so there is a non-zero chance to hit it with a "forward path" like that and it would be the only technique in Veach's sense to generate the path. Makes sense?
There is also some sort of "light leak" on the first mirror.
EDIT: Hm .. after some testing the strange "leak" appears to be no bug after all. I'll write it off as caustic cast by the mirror behind the one with the "leak".
Here is another shot. Moved the rear mirror a bit to see more of the caustics. Tuned down brightness. Converges and looks like normal illumination.