Here's Number Ten:
On April 1, 2003, as thousands of American-led coalition troops stormed across Iraq, the Iraqi ambassador to Russia, Abbas Khalaf Kunfuth, held a press conference in Moscow. Many were expecting him to announce that Iraq conceded defeat. Instead Kunfuth chose this moment to hold a gag press conference. Holding up a piece of paper that he identified as a news flash from Reuters, he read aloud from it: "The Americans have accidentally fired a nuclear missile into British forces, killing seven." Immediately the room full of reporters went silent with shock. Then Kunfuth grinned and shouted 'April Fools!' Only a few days after this unexpected moment of levity, the Iraqi government completely collapsed.
Read the rest. And here's a list of more successful April Fool's Day pranks.