With the release if the Maintenance Viewer (184.108.40.2062269) in week #11, invisiprims pretty much died their final death. Support ended for these critters in 2011. The code that renders them invisible has been in viewers up to this release. But, with this release something changed.
Inviziprims were prims with a special texture that caused things behind the texture to become transparent. Rendering-wise this was nightmare and problematic. The Lindens took advantage of a happenstance of the render pipeline to get them to work. I understand Crazy Mole once built a dry dock using invisiprims (the picture). Many boat builders used them to make the water inside their boats disappear.
The most popular use was with prim/sculpty shoes to make the feet disappear. Now we use alpha layers.
The new process is to use alpha layers. The render process is less costly and more consistent with other render engines.
No-Mod things that use invisiprims are obsolete and difficult if ot impossible to change. Toss them out and buy new. If you have the patience, you may be able to replace invisi-textures with clear textures. This would get rid of the gray blob that invisiprims have become, but clear won’t make the foot invisible. You’ll have to add an alpha-layer to the avatar to hide the foot.
It was the improved Advanced Lighting Model (ALM) that broke invisiprims. Or in a different frame, they prevented ALM from looking right. The invisiprim changed how the lighting with ALM rendered and made things look stupid. So, the process to render invisiprims was left out of the ALM pipeline. More gory etails here: Invisiprim. Basically, turn on ALM and it disables invisiprims.
If you run with ALM disabled, invisiprims still work, I think. But, people have started noticing a problem since the release of the latest Linden viewer and asking questions. So, something more has changed.