One of the most perplexing errors occurred in the production of the very common 2-cent Washington regular issue stamp of 1917. During the process of creating the plate, three of the images on the plate were judged to be defective, so those images were burnished out and reentered from a transfer roll. The transfer roll used, however, turned out to be for five cent stamps. Proofs were run from the modified plate, culminating in a certified proof containing 397 two cent stamps and 3 five cent stamps. The approval initials on the CPP said that the plate was perfect!
Large quantities of sheets of these stamps were printed, distributed to post offices and sold to the public before someone finally noticed that something was wrong. Postmasters were directed to return the sheets for destruction, but enough were sold that the error is not especially rare, compared to the inverted Jenny and other inverts. The error escaped notice for so long perhaps because the 2 and the 5 look similar, especially in the reverse image of the plate. Enlarge the CPP, then take a look at rows 8 and 9, fourth stamp from the left, and row 12 , third stamp from the right. What do you think?