A Krazy Couponer's currency IS coupons. Just as you would not use counterfeit money to pay for goods, so you would not use a fraudulent or copied coupon. This includes ALL coupons: Internet printables, manufacturer coupons from a newspaper or magazine, and store coupons.
So what happens if a copied or fraudulent coupon gets used, either purposefully or accidentally? While it could possibly scan at the register or get pushed through, when it reaches the reimbursement warehouse, the coupon will register that it is fraudulent or copied, and the manufacturer will refuse to pay the value. This means the store does not receive the money and must cover the cost themselves, just as if that money was stolen from the store. When this happens frequently, stores are more likely to put restrictions or bans on certain types and numbers of coupons allowed in an effort to stem losing this money. Using coupons correctly helps ensure future coupon acceptance, so we always want be honest in our couponing!