500 status codes are for unexpected server-side error scenarios. When an expired nonce is used by the client, a 4XX-level error is more appropriate because the client has submitted a bad request (by using an expired nonce). A 500 also causes Internet Explorer to show its default 500 page which does not show the error message and leads to a bad end user experience. I am choosing 400 for the new status rather than 401 or 403 because 401 requires a WWW-Authenticate header which would be difficult to generate in an SSO scenario and a 403 implies that no re-authentication will address the failure.
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