The interpretation of imaging studies is associated with missed diagnoses. These are due to the inherent limitations of the technique, which are quantified in the sensitivity and specificity of the test and can be caused by normal variation in the interpretation. This can be studied in the reproducibility. Another source of missed diagnoses is related to the observer and is the result of an inadequate technique, mistakes in perception and judgement, and a lack of knowledge. Here, perception error is probably the most important category. A reproachable error is made when a group of experts states that the error should not have been made. Some basic questions that can be posed to reach a judgement in such cases are: was the work conducted in accordance with a protocol, how were the films read and what was the clinical context? The missing of an abnormality on a film can only be judged against the background of the diagnostic handling around such a clinical problem and the quality of the observer.