Error notes are examples of paper money that somehow managed to make it into circulation, despite the obvious printing mistakes displayed on their faces (fronts) and backs. It shouldn’t be too surprising that a certain number of error notes exist. After all, the U.S. Bureau of Engraving and Printing produces more than 10 billion notes a year, so a statistically tiny number of errors is hardly an indication of lax quality standards. But that very scarcity is precisely what makes error notes so interesting to collectors.

Errors are often organized by when the error occurred during its printing and handling. U.S. paper money produced during the 20th century is printed in three waves. The first printing creates the image on the back of the note, the second printing is for the face, and the third printing adds serial numbers and seals to the face. Other errors are due to the way the paper is handled (folding and cutting errors) during any of these printings. In all cases, the more dramatic and glaring the error, the more valuable it is to collectors.

The so-called “king” of paper-money errors is a note with mismatched denominations on its face and back, which means the mistake occurred during the second printing (sometimes a ...mismatch error is perpetuated through the third printing, too). A less-valuable but related error is a note whose face is inverted. Double printings at any stage are also sought, evidence that a sheet has gone through the press more than once. The result is usually a note whose face, back, or overprints appear blurry, since the images are not perfectly aligned.