Medical students and experts were given head-and-shoulder photographs of patients, each showing a key feature of the patient's problem. Three quarters of these pictures were taken from textbooks. Noticing these supposedly obvious features was difficult and strongly influenced by contextual factors. Both experts and students gained about 20% in diagnostic accuracy by having the key features verbally described for them, although these were clearly visible on the photographs. Conversely, both experts and students reported seeing more of these features when the correct diagnosis was suggested to them. This facilitation resulted from an increase in sensitivity to depicted features, rather than a response bias. The properties of these features that allow such failures of noticing are discussed.