What is Diagnostic error?

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This question may look simple to answer but there are many pitfalls in reaching this conclusion
Research into diagnostic error, like research into the diagnosis process itself, is still a very new field. There is even difficulty in deciding what constitutes a diagnostic error. What a thoughtful patient may consider an error is not necessarily the same as that which his equally thoughtful doctor might consider an error.

For example, when a patient comes to my office with a sore throat and a fever, I might check for strep, and if it’s not present I’ll probably send him out with a diagnosis
of a viral illness. But I share with all such patients what I expect to happen over the next few days—that they should start to feel better within a day or two. And if not, I tell them to call me and let me know. Because, while the odds are overwhelming that this is simply a viral syndrome, it’s not 100 percent certain. I might be wrong. The test might be wrong. It might be mono. It might be some other kind of bacterial tonsillitis. It might be cancer. I can’t just check under the hood and see if the spark plugs need to be replaced—the way a mechanic diagnoses the funny noise your car is making. Instead, I have to listen to the engine and, based on the indirect evidence I can collect, make a thoughtful and well-informed guess as to what is probably going on. If I send that patient home with a diagnosis of a viral syndrome and he doesn’t get better and has to come back, would that be a diagnostic error? I suspect the patient would think so. And certainly it wasn’t a correct diagnosis. But did I make an error? Should I have done something different? I could have been more certain. I could have sent my patient to an Ear, Nose, and Throat specialist who could have looked down his throat with a special scope. I could have even asked for a biopsy of the red and swollen tissue to confirm my diagnosis. That would be time-consuming and painful for the patient and ridiculously expensive. But even then, the diagnosis would not have been 100 percent certain. In medicine, uncertainty is the water we swim in. The chance of being wrong is overwhelming when dealing with something more complicated than a sore throat. Doctors—far more than the patients they care for—recognize that some error is inevitable.

diagnostic errors—like this one nearly was—are often due to a multitude of missteps made along the way.
excerpt from

Every Patient Tells a Story, Medical Mysteries and the Art of Diagnosis – Lisa Sanders