Symptoms

After deleting choice columns from a SharePoint list and regenerating the SmartObjects, the SmartObjects associated with the choice columns are removed. When editing a workflow based on the library, the errors "SmartObject tcolumn name] could not be found." and "SmartObject does not exist on the server. Name: :SmartObject name]"
 

Diagnoses

This has been identified as a bug.
 

Resolution

To work around this, after deleting the column, save the workflow as a .wkprx file, open it in Notepad or similar app to view the XML. Locate and delete the applicable XML node(s), save, and redeploy the workflow.