In May, 2008, Robin Beaton, a retired registered nurse from Waxahachie, Texas, went to her dermatologist to be treated for acne. He mistakenly wrote down something on her chart that made it appear that she might have a pre-cancerous skin condition.

Not a big deal, right? It shouldn’t have been, except that soon after that, she was diagnosed with something far more serious–invasive and agressive breast cancer. Three days before she was scheduled for a double mastectomy, her insurance company, Blue Cross, called her and told her they were launching an investigation into the last five years of her health records. It turned out that dermatologist’s note had been a red flag, and the company was looking for a way to cancel her policy on the grounds that she had been hiding a serious medical condition.

What Robin went through after that was a nightmare, one she tearfully described Tuesday morning in front of the House Energy and Commerce Committee’s oversight and investigations subcommittee. “The sad thing is, Blue Cross gladly took my high premiums, and the first time I filed a claim and was suspected of having cancer, they searched high and low for a reason to cancel me,” said Robin, whose hair is just beginning to grow back in from chemotherapy.

The subcommittee took a look today at an immoral–and illegal–practice in which some health insurance companies engage. It’s called post-claims underwriting, and you should know about it. Because you or someone you love could be a victim if they buy insurance on the individual insurance market. Robin got her mastectomy, but only after her congressman, Joe Barton, leaned on the head of the company. (This is constituent service, in the very best sense of why we elect these guys. But the best thing they could do is to make sure it doesn’t happen to anyone’s constituent.)

There were other witnesses, too. Like Peggy Raddatz, whose brother Otto Raddatz lost his insurance coverage right before he was scheduled to receive an expensive stem-cell transplant to treat his lymphoma. Why? Because Fortis Insurance Company discovered that his doctor had found gall stones and an aneurysm on a CT scan–conditions that had nothing to do with his cancer, and that never bothered him, and that he wasn’t even aware of. And Jennifer Wittney Horton of Los Angeles, whose coverage was canceled because she had been taking a drug for irregular menstruation. Now, she can’t get coverage anywhere else. “Since my recission, I have had to take jobs that I do not want, and put my career goals on hold to ensure that I can find health insurance,” she told the subcommittee. “Fortunately, after my husband and I got married, I was able to gain coverage through his company’s group health care plan. However, if he ever loses his job, or I don’t have employment with a company that offers group health insurance, I might have to go without insurance.”

The insurance companies will argue that cases like these are rare, and that they have to be vigilant against fraud so that they can hold down costs for everyone else. But an investigation by the subcommittee found widespread instances where the insurance companies rescind coverage even over discrepancies that are unintentional, unknown to the policyholder or immaterial to the more serious health conditions for which the policyholders are filing for benefits. The three insurance companies called before the committee–Assurant (full disclosure: this company’s was part of a cover story I wrote for TIME about my brother), Golden Rule (a UnitedHealth subsidiary), and WellPoint–were a case in point. “The three insurance companies downplay the significance of these practices, arguing that recissions are relatively rare,” says Energy and Commerce Committee Chairman Henry Waxman. “But these three companies saved more than $300 million over the past five years as a result of rescissions. I am sure they view this amount as significant.” You can find a summary of the subcommittee report here.

Lisa Girion of my former employer, the Los Angeles Times, has done terrific work on this issue. Here’s what she reported today: