While healthcare providers strive for accuracy and thorough care, no one is above making a mistake. The interviewer understands that. The important thing is is being willing to address a mistake with professionalism and proper follow through. That is what the interviewer wants to see in your answer.
"I can recall an instance when some lab results were misfiled on a patient's paper chart. Unfortunately, the results were bad news which left the patient devastated. Once the error was realized, we called the patient and asked her to come into the clinic for a consultation. Of course, she was thankful to know that her prognosis was not bad, but it was a very emotional event. We used that experience as the foundation for implementing new guidelines on verifying and documenting orders before a patient was notified."
"I haven't been faced with an experience like this yet in my career, and I am very thankful to say so. I do always try to double check results, especially if they are indicative of an abnormality or poor prognosis before notifying a patient in hopes of preventing an episode like this."