The Danish Money Exchange
On March 11, 1980, the National Bank of Denmark issued a 20 kroner banknote that featured a picture of two house sparrows. Curiously, one of the sparrows appeared to be one-legged. This inspired the Roskilde Tidende
newspaper to run a story that year announcing that all bills with one-legged birds were fake, but that they could be exchanged at the post office for genuine bills depicting two-legged birds.
Lines at post offices soon became so long, with people eager to exchange their fake bills, that post office employees had to put notices on the doors explaining that no currency exchange was taking place.
The hoax was the brainchild of artist/cartoonist Jan Robert Thoresen. He was subsequently questioned by the police, but let go without any charges being filed since there was no law against improving the country's currency. Nor was it plausible that any government would ever allow people to swap counterfeit banknotes for genuine ones.