If you create a folder in a parent publication, but someone creates an item in that folder in a child publication, you cannot then delete the parent publication - the GUI gives the error "The item is in use".
The problem is that a Where Used wont show the fact that the folder has items within it. A user has to check the folder at each child publication - in larger blueprints this could be annoying!
Why can't the error message at least contain the context TCM Id of the folder from the child publication that is preventing deletion of the parent?