In 1976 in my home state of Ohio, they arrested a man in Columbus for counterfeiting postage stamps. He was found with 350 13cent liberty bell stamps (which were the first class rate stamp for the time). The fakes were nearly perfect, in fact the postal inspectors said the only things that were difference was that the color was lighter and the perforation was off.
The Liberty Bell 13 cent stamps were issued in a booklet form. Which means they would only be perforated on three sides at most. Not all four. The counterfeiter apparently didn't do his homework on this, and that is part of what allowed them to catch him.
Counterfeiting these stamps was not something that only happened here in Ohio. In fact they know that some were issued in Boston and Chicago as well. Sorta makes you wonder what possessed those individuals to counterfeit them. I wonder how often this happens today. I doubt they would counterfeit the new releases..though I could see where people would still counterfeit the older ones and sell them to people who use them for art. Or perhaps try to pass them off to people who are just starting to collect stamps.