That may have been a press check sheet and not a print error per se. 
Offset printers may occasionally use left over sheets from other runs as 
the check sheets for a new run. They typically get signed off by the 
press foreman and the press checker from the map author and then sent 
off with the checker. In some cases they probably make it into the print 
pallet.

That or someone fed a pallet of half printed sheets in as the blanks for 
a new print run.

David Medeiros

On 2015-03-24 09:02, Linda R Zellmer wrote:
> Hello,
> 
> While cataloging some maps this morning, I found what I think is the
> ultimate map printing error. One of our copies of the East Sheet of
> the 1974 Geologic map of the United States has topographic maps of the
> Indiana-Ohio border area printed on the back. I have attached a photo
> showing part of each side. Linda Zellmer