In 1962 the United States post office issued a stamp to honour the U.N. Secretary General Dag Hammarskjold. During the printing process one sheet got turned around so that the yellow printing was inverted. A stamp collector, Leonard Sherman, spotted the error at his local Post Office and bought the whole sheet for $2, expecting to make a lot of money from it. When the Postmaster General heard about it he ordered another forty million stamps to be printed with the error so that the original sheet was worth no more than the correctly printed stamps.