I Can't Sit on Things
If you find yourself unable to sit on a prim, it may be for one of the following reasons:
If the item you are trying to sit on is unscripted (or does not have a sit target defined) and you have placed it within the bounding box of another prim, such as a big piece of landscaping, then you will not be able to sit on it. You will need to either move the item out of the area of the prim, move the prim itself, or ask the creator to implement the sit target scripting correctly.
You can verify by taking the item up to a height where there are no other prims at all, and sitting on it.
If you find you cannot sit on anything (or being forced to stand up immediately), scripted or unscripted (or even groundsit), on one sim only, but other avies can, the chances are one item has a broken script and is using the “llUnSit” script command repeatedly in error.
To find out which item is preventing you from sitting:
Starting with the last thing you sat on in the affected sim, take the item to inventory. Once taken, try to sit on something else. If you can sit, the item taken has the broken script. You have the choice of either re-rezzing that item and resetting the scripts (if it is mod) or just not rezzing it again.
If you cannot remember which was the last item you sat on, take each item one at a time and try sitting until you have found the broken item.
If the item belongs to someone else, that person will need to reset the scripts or unrez it.