Just as a completely non-religious example, my girlfriend is a medical social worker and has been working recently with a patient that is absolutely convinced he has cancer.

There has apparently been some cases of tests that came back clean, and his excuses after the fact, but it's also turned in to him anticipating future tests not finding the cancer, yet with him still certain he has cancer. I had originally suggested that she ask him ahead of time something like, "well if we do this test and it comes back clean, will you let it go?" but he wasn't even open to the question.