The issue is only present in Firefox, there is a problem when remove / reinjecting a tinyMCE editor in a HTML textarea under certain conditions.

How to reproduce :
Drag and drop a content in edition at least twice (it works well the first time).
Another way to reproduce the bug is to change the tab display mode in edition twice for a content containing tab fieldsets.

In the firebug console, you can the the following error : NS_ERROR_UNEXPECTED: Unexpected error

Some link about this issue :
TinyMCE forum : http://www.tinymce.com/forum/viewtopic.php?id=18821
TinyMCE issue : http://www.tinymce.com/develop/bugtracker_view.php?id=3152
Possible patch : https://github.com/tinymce/tinymce/pull/175