When a user navigates a number of folders and files, the user may encounter files to which the user has read-only access.
This should not prevent the user from editing the contents, but does prevent the user from saving back the document to the original location.
The solution is to "force" the user to first create a read-write copy (that is not otherwise shared implicitly), clarifying the process to the user by forcing them to select a location to save the copy.