Symptoms
Error: "Invalid column name 'ColumnName_2' from SmartObject 'Create' method.
Diagnoses
Was not able to reproduce, thus I'm unconvinced that the issue was with the underscore naming convention or the amount of Memo fields. Likely a mismatch between service instance and SmartObject.
Resolution
Recreated the affected SmartObject in the SmartObject tester using the original's GUID.