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I know that at some point, German federal banknotes were only printed on one side.
Yep, printing on one side sped up the printing and saved money. The blank side of the notes came in handy as a source of good old scratch paper. German banks used to order blank pads of paper for their tellers to use as scratch paper, but during the worst of the hyper-inflation, bills became so worthless that the stack of money they sent as payment was larger than the stack of blank paper they got in return. Thus, at least in this circumstance, the banknotes were actually worth
less than the paper they were printed on. Banks took to just using the blank sides of the notes as scratch paper. They could throw the money away at the end of the day, they'd still be coming out ahead.