On March 31, 1940 Philadelphia radio station KYW broadcast the following message:
Your worst fears that the world will end are confirmed by astronomers of Franklin Institute, Philadelphia. Scientists predict that the world will end at 3 P.M. Eastern Standard Time tomorrow. This is no April Fool joke. Confirmation can be obtained from Wagner Schlesinger, director of the Fels Planetarium of this city.
The announcement came after a radio program by Jack Benny that had been devoted to a discussion of how the world might end. The program had mentioned the name of Orson Welles, who had been responsible for the notorious War of the Worlds Panic Broadcast of 1938
. The public reaction to KYW's announcement was dramatic. Newspapers, police stations and the city's information bureau received hundreds of calls from frightened citizens.
KYW later issued an apology and an explanation. The announcement was, of course, false, but the station denied responsibility for it. It said that it had received the announcement from William Castellini, press agent for the Franklin Institute and had read it in good faith, believing it to be genuine. However, Castellini had intended it as a publicity stunt to publicize an April 1st lecture at the planetarium titled "How Will the World End?" Castellini later explained that he came up with the idea for the stunt after hearing Benny's program and thinking it a good chance to get some publicity for the planetarium. He claimed, in his own defense, that he had told "some of the people" at the radio station about the announcement and "thought they would know it was a stunt." Soon afterwards, the Franklin Institute dismissed Castellini. [Oakland Tribune
, Apr 1, 1940; The Washington Post
, Apr 2, 1940.]
Not sporting at all, and as such I don't believe it should be on a top list, regardless of the scale or results. Maybe an honourable mention, though.
The best most succesfull April Fool's jokes are the ones that are very obvious.