From Heritage's website:
"This colorful error is commonly referred by those in the collecting fraternity as 'The Del Monte Note,' and the story about how this sticker got on this 1996 $20 Federal Reserve Note is as exotic as the Ecuadorian Banana sticker itself. The error is referred to as a 'retained obstruction,' or a note that was printed with a foreign object on the paper. Most obstructions fall off shortly after printing leaving a blank area of paper missing the design, but errors with objects that 'stick' to the note are very rare. Objects seen on other obstruction errors include a Band-Aid, paper fragments, scotch tape, and wood shavings."
"What makes this note truly special," Johnston continued, "is the stage of the printing process at which the sticker affixed itself to the note. United States Currency is essentially printed in three stages: the first printing is the back of the note, the second printing provides the face devices, and the third, final printing includes the Treasury Seal and the serial numbers. When this note was printed at the Fort Worth facility of the Bureau of Engraving and Printing, it went through first and second printings before the sticker found its way onto the surface. The sticker's placement is ideal, as it covers part of the second printing details and is overlaid by part of the Treasury Seal and serial number from the third printing."