Sounds like an embarrasing mistake by S&P but we must remember that governments are hardly angels when it comes to financial accountability.
As Ken Rogoff and Carmen Reinhart explain in their excellent “This Time is Different“:
One can only surmise that many governments do not want capital markets to fully recognise the risks they are running by piling on debt and debt guarantees due to their fear of having to pay much higher financing costs. Publishing historical data will make investors ask why current data can not be made equally available … Yet today even the United States runs an extraordinarily opaque accounting system, replete with potentially costly off-budget guarantees. In its response to the most recent financial crisis, the US Government (inluding the Federal Reserve Board) took huge off-balance sheet reserves onto its books, arguably taking on liablities that, from an actuarial perspective – as evaluated at the time of the bailout – were of the same order of magnitude, as, say, expenditurres on defense, if not greater.