Asami Novak has published a list of the "10 Best April Fools' Gags"...but they were published in the April issue of Wired...so maybe they are yet another April Fool trick! Nontheless, here are a few of them....
1969: Due to a moire effect, stretching pantyhose over the screen of a black-and-white TV will turn it into a color TV....so claimed technical experts at Sweden's only television channel.
1976: Pluto was to pass behind Jupiter at precisely 9:47 am on April 1, causing a brief reduction in the earth's gravitational effect. BBC Radio's astronomer, Patrick Moore, asked his listeners to all jump into the air at that instant to experience "a floating sensation." At 9:48 the station was besieged by calls from listeners reporting success!
1995: A special creature, the "hotheaded naked ice borer," was mole-like with a fiery, bony forehead. It lay hidden under the Antarctica, "melting the ice beneath the butts of hapless pemguins and eating them as they sink." Discover magazine had to print a retraction to dispel growing fears.
1997: The World Wide Web was to be shut down for cleaning between March 31 and April 2. The Japanese had built five "multi-lingula Internet-crawling robots...[to] remove electronic flotsam and jetsam."
1998: The Alabama Legislature voted to round the value of pi to 3.0, in order to correspond with the Biblical passage I Kings 7:23, which described the ratio of a bowl's circumference to its diameter.
1998: According to M.I.T.'s homepage, Disney Enterprises bought M.I.T. for $6.9 billion, with the plan to move its campus to Orlando and rename the School of Engineering to be the School of Imagineering.
2003: A lone gunman shot and killed Bill Gates at a charity event in Los Angeles. When 3 South Korean TV networks aired this story, panic occured on the Seoul stock exchange leading to a 1.5% drop or loss of $3 billion!
For the best April Fool's trick in mathematics, see Martin Gardner's creative effort.
Source: Selected from "10 Best April Fools' Gags," Wired, April 2008, p. 56