Mistakes in the printing of paper money can turn a low-denomination bill into a valuable and sought-after collector's item. The extreme rarity of these pieces of flawed currency only makes them all the more enticing to some numismatists, meaning those who collect and study money. Stricter printing processes in modern times have made these results of monetary freak accidents more rare and more collectable than ever.
The ultimate value of a misprinted piece of paper currency ---dubbed "error notes", according to the American Numismatic Association --- depends on various factors, mostly to do with the rarity of the mistake and the condition of the paper currency itself. Crisp bills with glaring errors usually command a higher price tag. Market fluctuations and the personal whims of a collector or buyer further complicate matters. Because of this, values for error notes can fluctuate from less than £65 to tens of thousands of dollars. However, due merely to its rarity, a printing mistake almost guarantees that the bill will be worth more than its face value.
Why do they occur?
According to the U.S. Department of Treasury's Bureau of Engraving and Printing, the current production process of U.S. dollar bills involves over 65 separate steps. That, coupled with the sheer volume of currency produced (2.6 billion £0.6 bills and 1.8 billion £65 bills in financial year 2009 alone), makes it almost inevitable for there to be some mistakes, no matter how strict the process is. Most of the time, officials catch defective notes during the inspection process, before the bills go into circulation. In these cases, officials issue a replacement note, also known as star note due to the distinctive asterisk placed at the end of the bill's serial number.
Types of errors
A wide variety of mistakes exists in printed currency. These include printing and cutting misalignments, printed folds, unbalanced margins and ink smears. A double-denomination bill ---for example, a £6 bill in front and a £3 bill in back--- constitutes one of the most glaring errors, as well as one of the rarest and most valuable. A single rolled digit on a bill's serial number or a slightly misplaced ink element exemplifies subtler mistakes. These types of errors would occur during the overprinting process, when the Federal Reserve seal and Treasury seal, along with the serial numbers, are printed on top of the bill in black and green ink, respectively.
Rarity of mistakes
When inspectors catch wind of errors, the Bureau of Engraving and Printing just destroys the misprinted bills. This makes the few misprinted bills in circulation extremely rare. Dramatic mistakes tend to be more rare still than small, inconspicuous ones. Big-time collectors can get their hands at times on uncirculated bills that command a particularly high price because of their crisp condition. According to Bruce Giese, a paper money collecting enthusiast, the U.S. error rate is about one per 100,000 notes. However, the federal government has not given official numbers on how often these mistakes occur.
Not really mistakes
Several historical developments have led the U.S. Treasury to issue bills that at first may indicate a mistake, but are not really so. For instance, some bills from the 1940s have "HAWAII" printed on the reverse. The federal government issued these bills back then as an economic defence measure after the Japanese attacked Pearl Harbor during World War II. You may have also seen old bills with a seal of a different colour from the usual green. These colours simply represent different types of notes; for instance, a brown seal represents National Bank Notes issued by banks chartered by the federal government. Other apparent mistakes may suggest something else entirely; instead of an error note, it might be a false bill.