With the Japanese attack on Pearl Harbor on December 7, 1941, the United States found itself preparing for the unthinkable: what would happen if Hawaii fell under Japanese occupation? One of the many considerations was what to do about the money.
As long as Hawaii remained under U.S. control, Hawaiians had to have full and free use of the currency of the United States. If the islands fell to the Japanese, however, the U.S. did not want all that currency to be in the hands of its wartime enemy.
The solution was a special brand of currency called Hawaii Overprint Notes. Regular U.S. currency was printed with the word Hawaii appearing twice on the front and once on the back. The idea was that if the money supply in Hawaii was taken over by the Japanese, the United States could devalue any money that said Hawaii on it, thus effectively denying Japan the use of it.
The invasion of Hawaii never materialized, and the plan to save the currency did not have to go into effect. Hawaii Overprint Notes are still good today at face value if you have one and wish to make a purchase, but they are highly prized by collectors. A $1 bill in perfect condition can fetch over $400 today.