Another poster printed on the back??? Is that a common occurrence with proofs?
That brings up an interesting question I just thought of...
If a poster has a unique (or somewhat rare) "oops" due to an error in the printing process, how does that effect the value or collectibility?
I ask because years ago I used to collect coins. Many coins, especially the older ones, would have minting errors. In some cases they were somewhat unique, though in other cases they would print off many with the same "flaw". Maybe something fell on the plate or caused them to print two obverses or reverses on the same coin, etc. This created different types for a given year - and there could be dozens. The rarer the type, in most cases the more valuable it was.
I believe mistake stamps also have the same appeal.
Is there a similar thing with posters?