Le Poisson d'Avril
Le poisson d'avril (literally "April's fish"), attempting to attach a paper fish to the victim's back without being noticed, is la version française of the April Fool tradition.
...and, no fooling, le premier avril est l'anniversaire de Louis la Vache.
Alabama Changes the Value of Pi: The April 1998 newsletter of New Mexicans for Science and Reason contained an article claiming that the Alabama legislature had voted to change the value of the mathematical constant pi to the "Biblical value" of 3.0.
Spaghetti trees: The BBC television programme Panorama ran a famous hoax in 1957, showing the Swiss harvesting spaghetti from trees. A large number of people contacted the BBC wanting to know how to cultivate their own spaghetti trees.
Left Handed Whoppers: In 1998, Burger King ran an ad in USA Today, saying that people could get a Whopper for left-handed people whose condiments were designed to drip out the right side.
Taco Liberty Bell: In 1996, Taco Bell took out a full-page advertisement in The New York Times announcing that they had purchased the Liberty Bell to "reduce the country's debt" and renamed it the "Taco Liberty Bell." When asked about the sale, White House press secretary Mike McCurry replied with tongue in cheek that the Lincoln Memorial had also been sold and would henceforth be known as the Ford Lincoln Mercury Memorial.
San Serriffe: The Guardian (UK) printed a supplement in 1977 praising this fictional resort, its two main islands (Upper Caisse and Lower Caisse), its capital (Bodoni), and its leader (General Pica). Intrigued readers were later disappointed to learn that sans serif did not exist except as references to typeface terminology.
Metric time: Repeated several times in various countries, this hoax involves claiming that the time system will be changed to a one where units of time vary by powers of 10.
Smell-o-vision: In 1965, the BBC purported to conduct a trial of a new technology allowing the transmission of odor over the airwaves to all viewers. Many viewers reportedly contacted the BBC to report the trial's success.
Tower of Pisa: The Dutch television news reported once in the 1950s that the Tower of Pisa had fallen. Many shocked and even mourning people contacted the station.
Wrapping Televisions in Foil: In another year, the Dutch television news reported that the government had new technology to detect unlicensed televisions (in many European countries, television license fees fund public broadcasting), but that wrapping a television in aluminium foil could prevent its detection. Within a few hours, aluminium foil was sold out throughout the country.
World's Greatest Practical Jokes