dependent dropdowns

I am working on ajaxing country and city dropdown boxes. Able to fill city dropdown on country selection. Having an issue while editing the node. No issue for country dropdown but if I am changing country and selecting some other city it is giving error message. There is something I am missing in allowed_values. Anyone knows how to tackle this issue.

I am putting together a site where I have a set of terms that need to be assigned to a node during node creation. In this situation when a user creates a node, a drop down list of terms are presented to the user. Each set of terms is dependent upon the users previous selection in the dropdown to make selection of terms less cumbersome and overwhelming since there are thousands of possible selection combinations (ie. Manufacturer->Make->Model). Then the terms selected need to be used to make up the title of the node (i.e Manufacturer->Cessna | Make->Floatplane | Model->180 )