Mercy Cabrera, an insurance agent with Sunshine Life and Health Advisors, speaks on the phone as she helps a person with information about an insurance policy under the Affordable Care Act at the store setup in the Westland Mall on November 14, 2013 in Hialeah, Florida.

The Obama administration has spent roughly $840 million on HealthCare.gov, including more than $150 million just in cost overruns for the version that failed so badly when it launched last year.

The Government Accountability Office says cost overruns went hand-in-hand with the management failures that led to the disastrous launch of HealthCare.gov and the 36 state insurance exchanges it serves.

GAO's report, prepared for a House Energy and Commerce Committee hearing Thursday, details a long series of management, oversight, and contracting problems that plagued the entire process, from risky contracting practices in 2011 through the botched launch last October.

A massive repair effort got the site stabilized by January, and despite the early failures, the administration ended up exceeding its enrollment target for this year. More than 8 million people chose plans through Obamacare, including the 36 states whose marketplaces run through HealthCare.gov.

All the same, GAO says, similar problems could arise again without structural changes in the way the government manages its contracts and spending.

Here are the key findings from GAO's prepared testimony: