The Dag Hammarskjold invert error of 1962 was the first invert error to occur on any United States stamps since 1918. Stamps with these errors might at first appear to be color errors, since it is indeed the color scheme that is different on the inverted copies. But they are in actuality invert errors due to the type of printing mistake that caused them.
The normal 4c commemorative honoring Dag Hammarskjold, late Secretary General of the United Nations, was first issued in New York on October 23, 1962. The stamp was printed on the Glori Press in a yellow, black and brown design on white paper, and 121,440,00 normal stamps were printed. In November of the same year some invert errors of this stamp were discovered. The invert occurred when some sheets of stamps were fed into the pres backwards. As a result, the yellow plate number on each misprinted sheet was inverted, and a white area was left around the illustration of the United Nations building. The white area left as background for the 4c denomination was also inverted.
There were at first only 400 of these inverts printed, but because the Post Office Department did not wish to produce error rarities, they immediately printed 40,270,000 Dag Hammarskjold stamps identical to the invert errors. As a result, the invert errors which at first had a potential collective value of hundreds of thousands of dollars are now worth 50c each. And it is virtually impossible to tell a reprint from an original unless it has a clear early date, but an invert error on a first day cover, proving that stamp was from the original printing and not from the reprint, was sold in 2005 for US $3,500.