When trying to rename the slug of a page and the new name is already used by another page it'll get you a keyerror. A check should be made if the new slug name is taken and display a notice to the user.
django-cms 2.3.3 and django 1.4.1
Could you please post the traceback (using dpaste or whatever)
You should just get a validation error in the form
#1604 - fixed.