1. What did you know, when did you know it, and who told you?
Sebelius and HHS have been vague at best about who exactly told Sebelius the website had software problems. The glitches became apparent in testing just two weeks or so before launch.
Contractors who worked on the site testified last week that the testing was delayed until the late date at the direction of the Centers for Medicare and Medicaid Services (CMS), the HHS division that runs HealthCare.gov.
Committee members are likely to press Sebelius on why that testing was not done sooner so that the problems it exposed might have been fixed or largely addressed before the launch.
2. Did you ever consider not launching Oct. 1?
Sebelius has claimed that HealthCare.gov had to open to the public by that date, but that assertion has been challenged by some who said she had the discretion to delay.
Critics have said that given the software problems uncovered in the last-minute testing, the secretary—and the goal of enrolling up to 7 million people in Obamacare insurance by March—would have been better served by delaying the rollout to sort out those issues and not address them on the fly.
Technology experts and others have suggested that HealthCare.gov should have been phased in one state or group of states at a time, to see how the system worked on a smaller scale and make fixes along the way. HHS also could have phased in parts of the site, letting customers shop for plans weeks before they were required to create accounts and apply for coverage.
(Read more: HealthCare.gov fixable by late November: Obama official)
HealthCare.gov's problems are said to stem at least partly from the requirement that users create accounts before seeing the prices they would actually pay, as opposed to estimates.
3. Why has no one been fired?
When it became clear that HealthCare.gov had serious problems—ones that were being downplayed in HHS's initial comments after Oct. 1—a number of Republican opponents of Obamacare began demanding that Sebelius resign.
She has adamantly resisted those calls for her head and has been supported by the White House, which doesn't want to look weak in the face of GOP criticism, and which would also find it difficult if not impossible to get a successor for Sebelius confirmed because of acrimony over the law on Capitol Hill.
But Sebelius, HHS, and CMS also have rebuffed calls that anyone below Sebelius be fired, including CMS chief Marilyn Tavenner, whose HHS division has taken responsibility for the launch and running of HealthCare.gov.
(Read more: Medicare chief: Sorry about that)
It was CMS' job to coordinate site components, which were developed by individual contractors, and to ensure they worked together properly as designed. They clearly haven't.
At the same time, HHS has not said whether it will seek rebates from contractors because of problems with the quality of their work.