Like your ObamaCare plan? It'll cost you

“I would expect that probably the majority of 2014 enrollees are going to be impacted pretty substantially,” said Milliman analyst Paul Houchens.

Let’s say your income is at about 150 percent of the poverty line—roughly $17,000 per year. The law says you don’t have to pay more than 4 percent of your income for the benchmark plan in your area. You chose that plan this year, and you’re getting a pretty generous subsidy.

Your plan wants to raise its rates by 5 percent next year—not great, but not the end of the world when you’re only paying about $50 per month out of your pocket. You like the plan, the premium increase doesn’t seem like a lot, and HealthCare.gov was a headache last time, so you just auto-renew.

Unbeknownst to you, though, new insurers have started offering cheaper plans in your area. Your plan is no longer the benchmark plan; a cheaper one is. So now your subsidy is based on the cost of that plan, not the one you have. This means you’re on the hook not only for every dollar of your plan’s 5 percent premium increase, but also for every dollar of the difference in price between your plan and the new benchmark plan.