Baden 9 Kreuzer error

Color errors are the great drama of early philately, and this is Germany's best. Baden's 1851 9 kreuzer should have been printed on rose paper, but some were printed on the blue-green paper meant for the 6 kreuzer, apparently because the printer used the wrong plate with that paper. For decades only four copies were known, until a fifth turned up in a US collector's album and was announced in 2019. That June a cover franked with the error sold at the Heinrich Koehler auction house in Germany for about €1.52 million including the buyer's fee. It looks like an ordinary numeral stamp until you know which color it should be, and that quietness is what makes it such a good story to tell a newcomer.