The Obamacare website may work for people buying insurance, but beneath the surface, HealthCare.gov is still missing massive, critical pieces — and the deadline for finishing them keeps slipping.

As a result, the system’s “back end” is a tangle of technical workarounds moving billions of taxpayer dollars and consumer-paid premiums between the government and insurers. The parts under construction are essential for key functions such as accurately paying insurers. The longer they lag, experts say, the likelier they’ll trigger accounting problems that could leave the public on the hook for higher premium subsidies or health care costs…

The Obama administration posted a document earlier this month indicating that insurers will continue to be paid through an “interim” accounting process — pretty much a spreadsheet and some informed estimates — until at least September. When the permanent system eventually goes live, it could lead to a massive correction that either exposes taxpayers to more costs or puts pressure on insurance companies to raise prices.

“We have the mother of all reconciliations coming,” said insurance industry consultant Robsert Laszewski, using the official term for the correction. “It may be that the administration will not be able to give us a credible enrollment number until then because we really need a reconciliation to accomplish that.”