Some also skip sports injuries and other common medical problems

There are also smaller categories of care that may be left off these plans. Research from Kaiser and the consumer advocacy group Families USA found that limitations on specific treatments were common. Details differ by plan, but examples include:

Joint replacement surgery

Cataract treatment

Hernia repair surgery

Treatment for any injury incurred while the patient was intoxicated

Injuries resulting from organized sports

Treatment for acne or moles

Treatment for chronic fatigue or pain

Immunizations

Limits on the amount of care covered

The plans tend to limit how much care they will pay for in a given year. That means that if you develop an expensive illness, your insurance could get cut off when you hit the limit. A typical short-term plan covers a maximum of $250,000 to $2 million in medical care, according to the Kaiser study. Major medical plans, which follow rules set by the Affordable Care Act, can’t impose any such limits.

Some short-term plans also limit how much they will pay per day of care, like a Lifeshield plan sold in several states, which advertises its “freedom to choose any doctor or hospital,” but pays a maximum of $1,000 of hospital bills per day, or $250 for an ambulance ride.

A pre-existing condition may disqualify you

A history of health problems, even relatively minor ones like allergies or acne, could mean trouble buying one of these plans. Some plans won’t cover any care related to such a condition, even one not previously diagnosed. Other plans charge people with prior health problems a higher price or just deny them outright.

Some other weird things

There may be other strange rules. A review of some plan documents from Families USA found an Illinois plan that would cover only hospitalizations beginning during the week — inpatient stays that began on the weekend would not be allowed except in rare circumstances.

Some plans had waiting periods for care. Cancer treatment, for example, is not covered in certain plans during the first month a person is enrolled in a plan, and no treatment for illness is covered in the first five days. That’s the kind of detail that might be easy to overlook when signing up for a plan if you aren’t expecting a cancer diagnosis.

“I would encourage someone who is looking for coverage just for a catastrophic need to really read the fine print,” said Claire McAndrew, the director of campaigns and partnerships at Families USA. “They often contain complicated exclusions that your average consumer could never predict.”

Insurers and brokers have an incentive to market these plans

If you buy insurance from a broker, it may not be obvious whether a given plan will be a short-term plan or a more comprehensive Obamacare-compliant plan, unless you ask. Both insurers and brokers who help connect people with the plans will have an incentive to sell them.