VAN D. writes: What would a $20.00 US bill that has been printed with the back on both sides with the front on top of one of the backs and the number 20 backwards on the bill be worth? The series is a 1990. It appears that that ink on the back had gone through to the front, making the bill have both sides on one side.
Your $20 note is an “offset” printing error or offset transfer, meaning that the rollers that pull the currency sheets through the press accidentally received an impression from the printing plates, because no paper was in the press. The rollers then applied the impression to the next sheet that came through, creating, what looks like notes with both images printed on one side. Because the inked roller has positive image the offset will be negative, that is backwards. If you look at the second impression you will see that all elements are reversed.This extra impression will continue until the bed is cleaned of all ink.
These error currrency notes are popularly collected and are judged by the darkness of the offset. As more sheets go through the press the offset becomes lighter and lighter. Condition also plays and important part as uncirculated examples are more in demand. Approximate value range: $80-$150.