On the http page, section on "create/update/patch/transaction" is a sentence about the reaction when no return preference is given. It is clear the server gets choice of returning resources or not; but the parenthetical mention of OperationOutcome is confusing. I think it is saying that returning an OperationOutcome is forbidden in this case. It would seem this should be a choice, and it should be encouraged to return OperationOutcome.