I have encountered a problem with the demo on page http://jqueryui.com/demos/draggable/#sortable
When I drag the "drag me down" item at the top of the list into the sortable list, it work only if a pause before adding another one. If you add new items too fast, an gap (empty placeholder?) appears and the whole thing hangs. See the attached screenshot.

I use Firefox 3.6 on Windows 7.