Garfinkel described it like this: "My name is Agent Jenkins. I'm an investigator with the secret service, working on a counterfeiting case. And it's tough. Last year, my office got a priority call from an economist at Stanford. The economist was looking at something called the money supply and velocity and both were increasing a little too fast. They just didn't add up. The economist finally figured an organisation was printing its own electronic money -- just like the US government does.
Banks "invent" money on a daily basis. You would have to counterfeit a great deal of currency (probably more than it out there right not) before you would start making a serious impact on the money supply. That said, enough counterfeit money may change the way people value money, and may cause inflation. -Thomas