For patients with prostate cancer, it is a common surgical procedure: a doctor implants dozens of radioactive seeds to attack the disease. But when Dr. Gary D. Kao treated one patient at the veterans’ hospital in Philadelphia, his aim was more than a little off. Most of the seeds, 40 in all, landed in the patient’s healthy bladder, not the prostate. It was a serious mistake, and under federal rules, regulators investigated. But Dr. Kao, with their consent, made his mistake all but disappear. He simply rewrote his surgical plan to match the number of seeds in the prostate, investigators...