Why do radiologists miss dancing gorillas?

There is something odd about this scan of a patient's lung. Have you spotted it yet?
How about the dancing gorilla on the right?
 It is not an everyday finding for radiologists, who are skilled at searching scans for tiny anomalies with potentially life-threatening consequences. But in one study, more than three-quarters of specialist tumour spotters were caught out by the greatest anomaly of their career.