As Congress readies legislation to repeal and replace the Affordable Care Act (ACA), Congressional Budget Office (CBO) estimates will play an important and respected role as they did in the passage of the law in 2010. We now know that many of CBO’s projections of important aspects of the ACA have significantly differed from actual outcomes. In this piece, I highlight CBO’s key past errors in projecting effects of the ACA. They can largely be grouped into two categories. First, CBO projected that the exchanges would be stable by now with more than twice as many enrollees as they currently have, rather than suffering from severe adverse selection in most states as they now are. Second, CBO projected that the ACA Medicaid expansion would be much smaller and less expensive than it has turned out to be.