Some diseases are so harmful that failure to diagnose them properly can be considered medical malpractice. If you are misdiagnosed for cancer, meningitis, stroke, heart attack, or an autoimmunity disease that then causes severe damage to you, then you probably have a case against your doctor.
It's extremely easy for a prescription error on the part of your doctor to hurt or even kill you. Duplicating existing prescriptions, prescribing high dosages, providing confusing instructions, or prescribing the wrong medication can all be grounds for medical malpractice.