Yikes. Enrollment in Obamacare insurance plans this fall was lower than the Department of Health and Human Services reported at the time. The difference was about 400,000 people, or about 6 percent of the total.

Conservative critics say it’s proof that Obama is “cooking the books,” just as they have claimed all along. Senior Administration officials swear they made an honest mistake. They've offered what sounds (at least to me) like a plausible explanation. But even if that explanation is accurate, the error would still be inexcusable. 

Here’s what happened, according to an account by Alex Wayne of Bloomberg News. HHS originally reported that 7.3 million people were enrolled in plans they had selected through the healthcare.gov and state-operated versions of it. But HHS did not reveal many details about the data. When staff from the House Republican Oversight Committee got a hold of the information, they noticed some weird numbers. They investigated and learned that the Administration had included 400,000 stand-alone dental plans as part of the total for health insurance. The real enrollment number, as of September, should have been 6.97 million. As of October, it should have been 6.7 million. 

How could HHS have made such an error? Administration officials told me that, during the spring, they used an internal system for tracking how many people were shopping and selecting new insurance plans. That system distinguished between health and dental policies, and didn’t count the latter towards the sign-up totals. But, as you may recall, that data didn’t provide a critical piece of information: How many people were actually paying for their plans. To get that information, HHS got information  directly from insurers and used it to produce the autumn numbers. When they did so, they didn’t think to break out dental plans—and ended up including them accidentally.

Charles Gaba, whose ACAsignups.net has been the official unofficial website for tracking Affordable Care Act enrollment, told Wayne that he was “appalled” to learn of the mistake. Sylvia Burwell, the new HHS Secretary, expressed similar sentiments on twitter: 

It’s worth noting that 6.7 million enrollment as of October is a perfectly fine number. Although it’s not as high as 7.1 million, it’s close to the 7 million of the original enrollment projections and actually higher than the figures the Congressional Budget Office predicted after the website problems of late 2013.

Even so, people are going to be more skeptical of HHS figures in the future, for understandable reasons. It’s one more stumble for a program that, for despite its very real successes, has had too many already.