Symptoms

Error when viewing form in User Task state: "SmartObject" property ID is a required property for selected method Load. Value must be set.
 

Diagnoses

The SmartForms Client Event had no Item Reference in use, but had an Item Reference passing rule set in the wizard that when removed would reappear upon next edit.
 

Resolution

Recreated the Client Event with the SmartObject reference and the form data loaded fine.