Because of the mistake, tax credits were calculated either too high or too low, and officials are asking people to delay filing their taxes until the problem is corrected. Affected people are now being contacted by phone and email.
The Obama administration says it sent about 800,000 HealthCare.gov customers the wrong tax information, and officials are asking those consumers to delay filing their 2014 taxes.
The tax error disclosed Friday is a self-inflicted injury that comes on the heels of what President Barack Obama had touted as a successful enrollment season, with about 11.4 million people signed up.
California, which is running its own insurance market, just announced a similar problem affecting about 100,000 people in that state.
The errors mean that nearly 1 million people may have to wait longer to get their tax refunds this year.
Another 50,000 or so who already filed may have to resubmit their returns.
Federal officials also announced a special sign-up extension for uninsured people facing the health care law's tax penalties