The failure to diagnose a condition is a severe misdiagnosis if a doctor fails to identify the condition. The majority of malpractice lawsuits are caused by the failure to diagnose conditions such as heart attack, breast cancer, appendicitis, lung cancer, and colon cancer. However, these are not actually the most prevalent undiagnosed conditions; see How Common is Failure to Diagnose? for a list of conditions with their undiagnosed prevalence estimates.
Note that failure to diagnose a condition can also arise in conditions where a person has no symptoms or only mild symptoms and so never seeks medical advice. In fact this is often the case with the early stages of the above-mentioned cancers.
However, once you seek medical advice for symptoms, a failure-to-diagnose misdiagnosis is possible. This can occur if the doctor finds nothing wrong, or if the doctor makes the wrong diagnosis of another condition. There are numerous conditions that tend to be under-diagnosed.