After three years of suffering with pain so intense he couldn’t work, and undergoing three back surgeries, Gary Duncan, then 47 and living in Claremont, Calif., received a diagnosis of Lyme disease. To see doctors who specialized in Lyme, he and his wife, Holly, went out of their insurance plan’s network. They thought their preferred provider organization limited their out-of-pocket costs to $10,000 a year, which they were willing to spend to make Gary well. They didn't know they were on their way to a $20,000 tick bite.

They knew that every time an out-of-­network doctor sent them a bill, their insurance would cover 60 percent. What they didn’t understand was that meant 60 percent of what their insurer deemed a “usual, customary, and reasonable,” or UCR, charge, which was often much less than the providers’ actual fees. So when a doctor charged them, say, $1,000, and the plan determined the UCR charge was $800, the plan paid only 60 percent of $800, or $480—not 60 percent of $1,000. Plus the plan had a $3,500 deductible. By the end of the year, the couple had paid more than $20,000 out of pocket.

If you're in the market for a new health plan during this open enrollment period, you could similarly rack up big bills if the medical providers you use are not in your new plan’s network.

A plan's in-network providers have agreed to accept an insurance plan’s contracted rate. (You still must pay deductibles, co-insurance, and co-pays.) But providers who are not in the network can charge you whatever they think is reasonable and bill you for the balance not covered by your insurer, known as “balance billing.”