A note that has an error in it when going through the printing process is usually caught by the Bureau of Engraving and Printing (B.E.P.). If an erroneous notes is caught, it is destroyed and replaced by a duplicate note. These duplicate notes have the same serial number as the original, erroneous note; the only difference between the note is that the last letter of the serial number is replaced by the star (*) symbol.
wikipedia.org dumped 2003-03-17 with terodump