Uniface Strike Coins
Uniface strike coins occur when there have been two blank planchets in the press at the same time. The other blank will obstruct the die on either the obverse or reverse side, which will prevent it from having that design on the coin. There are many different variations involving uniface errors.
In addition to having a 100% blank obverse or reverse, a coin can be struck off-center, with a blank planchet in the collar which will obstruct one side of the off-center. There are also mated pairs which have a combination of multiple errors which can include a side which is uniface.
Finally, there are uniface strikes due to a die cap which adhered to the die, forming itself in the shape of a die and striking blank planchets.