Poser 7 has various bugs relating to this. Users of one SR will call users of another SR wrong, so there's no use trying to describe the behavior across the board. I have also occasionally seen this in Poser Pro and Poser 8, but at completely different times than Poser 7.

In P7, prior to SR3, I could get this to happen every single time I loaded a material that had an empty map in it.

The fundamental premise of VSS involves loading shaders without maps in them, so there's no easy workaround other than upgrade your copy of Poser.

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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)