Try this: Colonize a large quality planet (e.g., PQ 19) and designate an improvement for all of the buildable tiles. Now select one of the improvements (near the top is good) and click the down arrow. The item is moved down one slot but the original slot for this improvement is unchanged. If you try to delete the item (the one still selected but in the new slot), the improvement originally in this slot gets deleted from the queue. In addition, the tile for the original contents of that slot retains the image of its original "being built" project and will not accept a new project. You have to use the "destroy building" button to clear the tile in order to queue an improvement to the tile.
Another querk of the social improvement queue is when you try to move items up in the queue. The view is shifted to the top each time you click the up arrow, requiring you to reposition your view with the scroll bar. This makes it difficult to see where you are positioning the improvement in the queue.
