I made a chair with a simple sit script and put 4 chairs around a table on a platform in my sim 'Blue'. Standing and sitting alone seems ok but when another avi joins in things get wierd. The second person to sit sees the first swing round and lay on the table. If they then stand and sit again they see the first now laying on the table. Variations sometimes happen, like both laying on the table and even both sitting properly occasionally.
There is a bug in the actual stand process too. When the avatar stands it seems to be placed at ground level. As the chair is on a platform that means that the avi is now intersecting the platform and rushes off to the side to get free of it. Either that or flies up into the air.