Ink Error on dollar bill

My grandma was a huge coin and bill collector. When she passed away I inherited her collection. I had all the coins checked out, but this was the only bill they didn't know anything about. I am simply trying to figure out what it is and how it was ever in circulation with such a large error on it. The series is 1985 and about a quarter or a third of the back of the bill is solid green ink. It does not bleed over to the front.