The "Treskilling" Yellow, or “three schilling banco error of color” is a Swedish postage stamp of which only one example is known to exist.
In 1855, Sweden issued its first postage stamps in a set of five values ranging from 3 to 24 Swedish skillings. The three-skilling stamp was normally printed in a blue-green color, while eight-skilling stamp being printed in yellowish orange.
However, an eight-skilling printing plate was mistakenly replaced with a three-skilling. Hence, the new 3 skilling were printed in yellowish orange. The error went entirely unnoticed. The stamps were still in use till 1873 when the Swedish currency was changed.
In 1886, a young collector named Georg Wilhelm Backman was going through covers in his grandmother's attic. He came across one with a three-skilling stamp and was offered seven kronor apiece.
After that, the stamp changed many hands. As no other yellow surfaced in few days, it became clear that the stamp was not only rare but quite possibly the only surviving example. The stamp exchanged many hands at an escalating prize.
Recently, in 2013, the stamp was acquired in a private sale by Count Gustaf Douglas for $2,500,000.
Image Curtsey: https://en.wikipedia.org