“I’M GOING to try to download every movie ever made,” TV host Jon Stewart, opening a laptop, said to Health and Human Services Secretary Kathleen Sebelius this week, “and you are going to try to sign up for Obamacare, and we’ll see which happens first.”

It’s been an open secret that HealthCare.gov, the federal government’s insurance marketplace, would have some technical problems after opening on Oct. 1. But unfortunately for the millions of uninsured the Web site is supposed to help, what happened took some observers — and the Obama administration — by surprise.

Many of the millions who tried to access the Web site in its opening days were diverted to a virtual “waiting room,” where they couldn’t do anything useful. Once they did gain access to the site, it sometimes crashed as people attempted to create accounts, the first of three steps to getting coverage. At other stages in the process, some users got inaccurate information on whether they are eligible for government help in buying insurance or for state-run Medicaid programs. The sign-up information that insurance companies got from the federal interface also hasn’t always been complete, so they’ve had to fix applications by hand.

Administration officials have explained that too many people were visiting HealthCare.gov at one time. About 10 million unique visitors came to the site in its first week, a lot more than expected. At one point, the number of people trying to access it was five times the number it was built to accommodate. That’s a problem, especially when your Web site isn’t put together well. Information-technology experts have panned the site’s design, pointing to “stray code” and various other basic flaws that make Web sites balky and unstable.

HealthCare.gov isn’t meant to serve every would-be insurance buyer in every state. Some states decided to cooperate in the implementation of the Affordable Care Act and set up their own online marketplaces. Some of their sites also had problems, but many others worked reasonably well and have reported encouraging enrollment figures. More than 28,000 Californians and more than 40,000 New Yorkers have purchased insurance, long before the first coverage deadlines. Federal officials won’t say how many people have successfully signed up through HealthCare.gov. Republicans who refused to allow their states to create online sites almost certainly contributed to the technological breakdowns on the federal interface, which had to pick up the slack.

That doesn’t give the administration a pass, though. The federal site simply wasn’t ready to handle the phase-in of President Obama’s signature legislative achievement.

The encouraging view is that the overwhelming number of visitors to the site indicates high demand for insurance in the new system. That’s critical, because the more people who participate, the better the system works. The danger is that the technical foibles since Oct. 1 will discourage people from coming back, particularly the young and healthy customers the system needs to keep rates reasonable.

With some big deadlines approaching, the Obama administration needs to fix the bugs, and fast. People who need insurance from the federal marketplace have until mid-December to buy coverage that begins Jan. 1. If enrollment figures aren’t robust by then, the new health-care system will have a problem even bigger than a frustrating Web site.