The double denomination is the king of all error notes. These are rarely created by the Bureau of Engraving and Printing. In fact I have never seen one for any of the modern big face series.
Double denominations have some mix of one denomination on the front and another on the back. You most often see a five and a ten. However, most any combination is possible.
It is impossible to show what a double denomination looks like via a scanned image, because you can only take an image of one side. The image below is not what an actual double denomination looks like.
If you have a bill with two different denominations, send us an email and tell us about it. All double denominations are very valuable and we are aggressive buyers of such misprints. firstname.lastname@example.org