Symptoms
Workflow event outcome with conditional Inline Function not passing a test as expected and returning an error: "Object of type 'System.Object[]' cannot be converted to type 'System.String'."
Diagnoses
The Inline Function condition was a simple Contains() Inline Function, checking for a string value from the parent list's SmartObject reference in the workflow. The field was generated off of a lookup column to another list and looked to be returning multiple values in a single field.
Resolution
Changed the structure of the condition to include a Join() Inline Function, to join the multiple values into a single string for the Contains() function.