When is a fake a forgery? When is it a mistake? And when is it simply not what it appears? The National Gallery’s second summer exhibition to focus on its own collection here examines the questions of attribution, using the latest scientific resources to back up – or contradict – tradition, connoisseurship and curatorial decisions, good and bad. The gallery is putting its own mistakes on show, and over the 170-plus years of its existence, there have been more than a few.