Here is a simple example that my sister (a nurse) gave me yesterday. Suppose that you go in for an operation at the hospital, spend a week recovering, and develop diarrhea on day 2 while you are there. That diarrhea is a "hospital acquired infection" and insurance won't pay a dime for your operation. Therefore until you've been through the hospital, nobody knows whether you'll get paid.
Oh right, and the possibility of this happening is a reason for the hospital to kick you out of the hospital as quickly as possible. Average patient outcomes may be better if you stay a week, but their odds of getting paid are better if you're kicked out within 48 hours.
This is just the tip of the iceberg. She went on about how broken health care is for an hour...