CHICAGO (AP) -- Many Americans are finding out that free preventive care under the nation's health overhaul isn't always free.

Sometimes doctors are confused and charge a copayment when they shouldn't. Other times a screening test turns into treatment. An Arizona man wound up paying $1,100 after his free colonoscopy revealed a polyp, a possible warning sign for cancer. A Florida woman had to argue with hospital staff to avoid paying $700 for a mammogram.

President Barack Obama's health overhaul requires most insurance plans to pay for cancer screenings, cholesterol tests and other preventive services. Millions of patients have received such services this year.

But a few expensive surprise bills has some screening advocates worried that such experiences may undercut the law's intent of preventing more serious health problems by catching them early.