The link to the bug in JIRA is: http://jira.secondlife.com/browse/SVC-3806
Also a duplicate entry of the bug report with further information can be found at: http://jira.secondlife.com/browse/SVC-3811
The Summary of the Bug States:
Sit is disabled on prims (without a sit target) that are inside a sculpted prim's bounding box, and it can be much bigger than the visible sculpt: see attached image.
The same happens on prims that are inside another phantom prim.
This can inhibit sit on large areas, with content break and security (sitting shields you from many attacks, immediately) implications.
Unfortunately there's little I can do to fix this bug as it has nothing to do with my furniture.
DON'T PANIC. At the present time any items I have with sit scripts for only one avatar are FINE. The items that are affected are the new beds with multi avatar multi sit scripts v.1.1 and the bow arm couch version 1.6.
If you have these items in your home that's close to say a huge prim volcanic rock from Heart Botanicals you may notice that in the current release version of the SL client, you are no longer able to sit / lay in these items.
I think patience will prevail, we have to give Linden Lab enough time to fix the bug.
Thanks for your patience.