Spotted some "related" weird System Skirt Alpha interaction Problem - See pics ..
Wear any system skirt and stand on a prim with any transparency / alpha (I simply rezzed a prim and set it to 2 % transparency) .. with Lighting and Shadows Selected .. there looks to be a "alpha system skirt" going down to the ankles .. Turn off Lighting and shadows and its OK
related to http://jira.phoenixviewer.com/browse/FIRE-1528 ???
Edit (Whirly): https://jira.secondlife.com/browse/VWR-28533