For some months now there has been a problem of some prims not rendering. You can walk into them or on them. But, you cannot see them until you right click on them. With this current server release the problem seems to have gotten worse.
Inara Pey has an article up with more details: Missing” prims: collaboration confirms viewer issue.
She quotes Maestro Linden explaining how to troubleshoot. There are a couple of things in the explanation that will be handy for SL users to know.
Things Failing to Rez Correctly
If you run the main SL Viewer, Beta Viewer, and/or Dev Viewer, you may run into an additional problem where your cache corrupts. Mesh items can suddenly turn into Vertex Vomit, something sculpties first did. The object looks more like a porcupine than what it is supposed to.
I think I ran into this problem because of using so many viewers. I allowed all the SL Viewers to use the same cache. It looks like that was a mistake. I saw the problem when I changed to the new Mesh Deformer Project Viewer. I doubt that it caused the problem as there could be any number of things that might corrupt the cache. But for now I am making separate caches for the project and development viewers.
This problem is related to various optimizations in the render pipeline and to race conditions. There is no simple fix (short of rewriting a lot of code), and I instead implemented a work-around for this issue in the Cool VL Viewer. The result of the bugs is that the vertex buffers got missing data (the data that would be needed to render the missing prims), so the work around is to reset the vertex buffers (so that they get refilled) shortly after the TP (this is what the Cool VL Viewer is doing). I also implemented a manual way to reset the vertex buffers (in the Advanced -> Rendering menu) for cases when lag is so bad that the auto-resetting comes too soon.