The travel health insurance that millions of Canadians buy may offer no protection at all if they answer a single question incorrectly — no matter how innocent the error — on a medical questionnaire full of convoluted language, confusing clauses and tricky definitions, the CBC's Marketplace has learned.

For some people, the result can be financial ruin when their claims are denied.

That’s what happened to Bill Jennings, 67, and his wife Tracy, 48, of Gold River, B.C., who bought travel insurance from Manulife before leaving on a trip to Fort Walton, Fla., in December 2010.

LIVE CHAT Going on vacation? Think travel health insurance is going to protect you if something goes wrong? Join Marketplace host Erica Johnson and insurance lawyer Scott Stanley on CBCNews.ca answer your questions about how to avoid the pitfalls of buying travel health insurance.

They filled out the application, including the medical questionnaire, online and thought they were covered in case of illness during their trip.

In Florida, Bill had chest pains and numbness in his arm. He discovered he had suffered a heart attack and needed emergency surgery to remove five blockages in his heart.

The surgeon explained that Bill could not be flown home because a change in altitude could bring on another heart attack.

"Which is why we bought insurance in the first place — unexpected emergency," Tracy said. "This was unexpected."

Recovering back home, Bill was stunned to receive a letter six months later, saying his travel health insurance claim was denied and he owed $346,000 US in medical bills.

"I was just in shock," he said. "I’ve always had travel insurance whenever I’ve left the country. Just in case something happens."

Added Tracy: "We're in financial ruin."

The problem, according to his insurer, Manulife, was the way the Jennings filled out the medical questionnaire.

Watch Tripped Up on CBC's Marketplace on Friday at 8 p.m. ET, 8:30 p.m. in Newfoundland

Manulife says Bill should have answered yes to this question about two conditions:

"In the last two (2) years, have you been prescribed or received treatment for and/or been hospitalized (as an in-patient or seen in the emergency department) and/or been prescribed or taken medication for any of the following conditions: diverticular disorder or gastrointestinal bleeding?"

Bill insists that he didn’t know what was spelled out in his medical file or that he’d been diagnosed with those two conditions. He thought all his symptoms were related to the colon cancer he’d had surgery for 19 months earlier.

"Most importantly to me would be the question, 'What does anything, what does anything related to this have to do with Bill’s heart?'" Tracy said. "Absolutely nothing. Absolutely nothing."

Susan Eng of CARP, a Canadian advocacy group for people over 50, says the system is set up for claims to be denied.

"Ordinary people are out thousands and thousands of dollars because they did not get the protection they thought they had — only because they made a mistake on the form that they could not possibly have done correctly," she said.

While people do have the responsibility to fill out the medical questionnaires correctly, the bar is set too high, she said.

"They have to do their best. But as we’ve looked at some examples, the best is not good enough. They simply have all the cards stacked up against them."

According to David Hartman, president of the Travel Health Insurance Association of Canada, of the 500,000 travel insurance claims made every year, 15,000 are denied for a variety of reasons. There are no industry numbers on how many of those denials are because of the medical questionnaire, but research by the CBC's Marketplace shows the number could be in the thousands.

Bill collectors are pursuing the Jennings, though there is a glimmer of hope. After the Florida hospital learned the couple were uninsured, it cut their bill. But they still owe $160,000 US.

"My biggest fear is seizing of our property, taking our home," Tracy said.

Insurance companies have had the upper hand for far too long, she said.

"Bottom line is, there’s something wrong in the industry when people are planning a trip, think they have all their t’s crossed and their i’s dotted. We get home, and now we’re in financial ruin. Something should be changed in the industry to protect consumers."