As far as Poser Firefly rendering is concerned, there is more than one way to get double sided artifacts. Whether there are two faces sharing the same vertices, but with opposite winding orders; or whether there are two distinct faces with independent vertices, but coincident in space, the effect is the same. And again, as long as the inner and outer faces are moved apart, the artifacts are avoided; this can be done either by modelling the mesh with "thickness", or in the renderer by adding a small displacement to one face. DAZ|Studio doesn't mind double sided facets, by the way, it's just Poser - and only when using the Firefly renderer, at that.

Quote - Isn't there a note somewhere about only doing cloth simulations "with single-sided, uncapped objects"

Sure there is. Here it is again. Don't do cloth simulations with double sided objects, or with objects which intersect either them selves or the figure they collide against. :)

I have a hypothesis that the cloth room is coded to believe that the "inside" of a single sided mesh is the only thing that should be involved in figure collisions - so it gets confused when faced with the outside of a facet, as I discovered when trying to cinch clothing in with a belt, for instance. This also implies that clothing meshes should be modelled with their outsides facing the right way, regardless of whether you can get them to render or not, although I haven't tested this. The cloth simulation isn't accurate enough to "roll" a mesh with thickness down the figure, perhaps unfortunately. :)