You're not allowed to print fake money that looks like too much like real money. See https://www.marketplace.org/2015/03/10/business/tricky-busin....

From the article: "Essentially what this law says is that bills must be either 75% smaller than or 150% larger than the size of a real bill and one color, one side."

So printing fake money could be a crime even if you don't attempt to pass it as real.