I have experienced this problem only on Internet Explorer.
I have a page with a sidebar and main area.
The main area is updated via Ajax calls from links in the sidebar.
When the main area is updated and removes elements that have been referenced in a call to "drop_receiving_element" (rails)
drags of objects from the sidebar fail with an error.
I have tracked this error down to a problem when enumerating through the list of droppable objects.
I can "resolve" this problem (sorry, HIDE it) by altering the prototype Enum.each method to drop all errors (have not got time at the moment to investigate, when I do, I will post more information.)