MM1's glass marbles

Your marbles may be reflecting the background via the new marble shaders in PQ. Easiest method to stop this is by changing the texture file name to something that doesn't contain marble (horrible fix for a horrible hack). Then it should just render as normal.

Yup, that's exactly what was happening. It only affects marbles whose texture names end in .marble, and disabling the reflections in the Options menu fixed everything. I'll add a note to the original post - thanks!