Misprinted Bill with the Back Printed on the Front Offset
A full offset misprint occurs when wet ink is transferred to either the face or back of a different bill. Full offset misprints are actually rather common. They can be purchased in most situations for $50 to $250 depending on the denomination of the bill.
If the offset affects something like a $2 bill or an earlier issue like a red or blue seal note, then an additional premium should be added.