There is a same ticket about this. detials http://dev.jquery.com/ticket/1699. But that user couldn't reproduce it.

Well I can reproduce it. I think I know why its happening.

it happens when I drag a draggable which is already in a droppable.

but I want to move the draggable from one droppable to other droppable.

I was trying to create a page like http://www.google.com/ig

see the gadgets there. all gadgets there are draggable. and in the same time they are in a droppable content.

the following structure will produce this error.

<div class="droppable"></div>
<div class="droppable"></div>
<div class="droppable">
  <div class="draggable"></div>
</div>
<div class="droppable"></div>
<div class="droppable">
  <div class="draggablecontent">
    <div class="draggable"></div>
  </div>
</div>

for each draggable in the above structure, it will produce the error. If you want I can show you my source. But I think the above fragment is enought.