Treskilling Yellow

The archetype of the unique error. Sweden's 3 skilling banco of 1855 was printed in pale bluish green, yet one copy exists in yellow, the shade used for the 8 skilling. A young Stockholm collector, Georg Backman, discovered it in the mid-1880s and sold it a few months later for 7 kronor. Experts once declared it a forgery, a cloud that only deepened the legend. Between 1984 and 1996 it sold three times, setting a world record price for a postage stamp each time, and the 1996 sale realized 2.88 million Swiss francs. One small yellow square, and philately's most famous question of how it happened.