Machine Doubling. Remember when you have to look at the edge of the devices, that is where Machine Doubling lives. A doubled die show a spread on the centers of the devices. Not on the edges except for notching.
When you find a doubled die, they will leap out at your. Machine Doubling you find because you spent too long on the coin. When you find your first doubled die, then you will realize you search to hard for something that wasn't there on your coins. But when you do find one, the you will remember this post.