Between the years 1920 and 1950, the two dollar bill was prevelant. All barbershops were historically closed on mondays, a good day for a lot of barbers to go to the racetrack. Racetracks also paid off with two dollar bills. When the day was over and the barbers went home, if they forgot to remove the two dollar bills, and their wives saw a two dollar bill in their wallets, the fights began. The wife would know the husband had been at the track. So barbers considered them unlucky and that superstition has just passed forward.