Never...and if you take note...when it involves something from the Dems they are optimistic...if it is the GOP they are predicting a disaster...when in FACT it is usually the other way around
When you're scoring huge expensive bills with incredible complexity, perfect accuracy cannot be expected. They typically have good ball-park figures. That's all anyone can realistically expect. Many things beyond anyones control change the circumstances. In the case of the Recovery Act, for example, they made their estimate based on the conditions in existence at the time. It took long enough to pass the bill that hundreds of thousands of additional people lost their jobs increasing the cost of extra weeks of unemployment insurance funded by the bill. Given the complexity of scoring that particular bill, the 10% margin of error they seem to have achieved is pretty good.
I don't follow the CBO. However, it concerns me that congress forms policy based on CBO numbers. I usually go to the Federal Reserve website in the zero reports to get real numbers on deficits and GDP.