After checking all the possible actions, if none match and the request's action ID is not null, the adapter can throw an AuthnErrorException. The adapter catches this exception and writes an error to the API response.

if (apiSupport.getActionId(req) != null)
	// An action ID was provided but it does not match one of those expected in the current state.
	throw new AuthnErrorException(CommonErrorSpec.INVALID_ACTION_ID.makeInstance());