The Baden 9 Kreuzer Error is a postage stamp error produced by the historical German state of Baden in 1851. Baden's first postage stamps were issued on 1 May 1851. The "9 Kreuzer Green" stamp was a color misprint of the 9 Kreuzer denomination that was printed in green instead of pink. The green color was intended for the 6 Kreuzer value, but apparently, the paper sheets were mismatched. Only three cancelled copies and one unused copy of this error are known, but more sheets of paper may have been printed. The cancellations recorded have the numbers "4" for Achern, "41" for Ettenheim, and "106" for Orschweier (today Mahlberg). Two of the known copies are on letters.

Copies

The error is one of the greatest philatelic rarities in the world. The 9 Kreuzer error was not discovered until 44 years after the stamp was issued. Two letters initially were in the collection of Baron von Türckheim. The usual explanation for the occurrence of this error is that the printing plate was accidentally inverted; however, this theory cannot be correct because the stamp was produced in a single printing. It must be assumed that the printer had unintentionally used the wrong plate for the green paper: instead of a "9" he had read a "6".