This scarce and prominent variety of the 1942 Mercury dime was created when a die used to strike the dimes was first struck with a 1941 hub, then later struck with a 1942 hub. This was, in all likelihood, a mistake - not an intentional cost-saving measure. Experts believe that a single 1941-dated die got mixed in with undated dies being prepared for the next year's production. This famous 2 over 1 variety has become a key to completing a Mercury dime collection.