Just about every denomination of US currency has been counterfeited at some point. But there are some really good copies out there (aka supernotes
) that are extremely difficult to detect. In response, every few years, the government has to issue new bill designs that are harder and harder to duplicate. Here are just a few interesting links on detecting fake money and issuing new currency.
If you'd like to read more awesome and interesting stuff, check out this unrelated (but not entirely random!) Techdirt post