Jim Pethokoukis offers a wonderful example of CBO 10-year projection failure: in 2002, the CBO projected that debt would be 7.4% of GDP by 2012. The actual figure: closer to 74%.
Did a lot of unexpected things happen between 2002 and 2012? Of course they did. They always do. This is precisely why you should never regard 10-year budget forecasts as “facts.” It’s why I apply what I call Crank’s First Law: government budget and financial forecasts are always, always wrong.