A growing body of research suggests that this nightmare scenario is fairly common for Americans. And now, a new study published in the Journal of the American Medical Association has found that surprise bills might be even more common than previously estimated: They happen about a fifth of the time that a patient has an elective surgery at an in-network hospital with an in-network surgeon. Having a surprise out-of-network bill raised the total bill by an average of $14,083. The dollars racked up while many patients were unconscious, and an out-of-network specialist simply walked into the room.

For the study, the authors looked at 347,356 people nationwide who were insured with a major private insurer. (The authors declined to name the company.) By reviewing bills from seven common elective procedures undergone from 2012 to 2017, including the aforementioned knee-cartilage procedure and hysterectomies, they found that the surprise bills came most often from anesthesiologists, who put patients to sleep before procedures, and surgical assistants, who do everything from aid in an operation to check on the patient afterward. Though the hospital might be in-network, these specialists might not work for the hospital, but simply treat patients there. The out-of-network surgical assistants charged more, at $3,633 on average, than did the anesthesiologists, at $1,219.

To the University of Michigan researcher Karan Chhabra, the lead author of the study, the most surprising thing is that surgeons can often choose their own assistants, but not their anesthesiologists. So why might surgeons pick out-of-network surgical assistants? “My hope is that it’s an accident,” Chhabra told me. “[Doctors] don’t always talk about which insurance everyone accepts.” Of course, most doctors are acting in good faith, but, he added, “my concern is that in some cases it might be happening intentionally to sort of exploit patients.” For example, surgeons might be teaming up with out-of-network assistants, and vice versa, to get more money from patients.

Karen Pollitz, a senior fellow at the Kaiser Family Foundation, told me that though this study highlighted anesthesiologists and surgical assistants, surprise bills can also come from out-of-network pathologists, who analyze tissue and blood samples, or radiologists, who examine X-rays and MRIs. No matter who the surprise bills come from, “you don’t pick these people. You don’t know them,” Pollitz said. “You learn their name when the bill comes.”

The results in the new JAMA study are similar to, though slightly higher than, findings from the Kaiser Family Foundation. The health-care nonprofit recently found that about 18 percent of emergency visits and 16 percent of inpatient admissions result in surprise out-of-network bills. People who go to the hospital with a heart attack are especially vulnerable to surprise bills, it found.