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Please note that a service consumer should not assume that an error message payload is always returned by the endpoint provider. Some implementations are more vigilant than others and may consistently provide an error message payload while other implementations may not. There is a potential that some proxies or other network filters may remove payloads from a HTTP response for certain HTTP error status codes. A typical case where it is unlikely that a SIF structured error payload is returned in a response is the HTTP 404 (Not Found). This error code is typically returned if an incorrect URL is used. In such a case the actual endpoint provider is not reached and hence a SIF error message payload is not produced.
Software providing error payloads shall not include actual data within the message. It may be specific as to what field has an issue, but not what was supplied. This will help prevent both fragile workflows and privacy violations as none of the usual validity or privacy checks can be applied to data included in the structure defined here.