The discoverer pitches the claim directly to the media.
The integrity of science rests on the willingness of scientists to expose new ideas and findings to the scrutiny of other scientists. Thus, scientists expect their colleagues to reveal new findings to them initially. An attempt to bypass peer review by taking a new result directly to the media, and thence to the public, suggests that the work is unlikely to stand up to close examination by other scientists.
One notorious example is the claim made in 1989 by two chemists from the University of Utah, B. Stanley Pons and Martin Fleischmann, that they had discovered cold fusion -- a way to produce nuclear fusion without expensive equipment. Scientists did not learn of the claim until they read reports of a news conference. Moreover, the announcement dealt largely with the economic potential of the discovery and was devoid of the sort of details that might have enabled other scientists to judge the strength of the claim or to repeat the experiment.
(Ian Wilmut's announcement that he had successfully cloned a sheep was just as public as Pons and Fleischmann's claim, but in the case of cloning, abundant scientific details allowed scientists to judge the work's validity.)
Some scientific claims avoid even the scrutiny of reporters by appearing in paid commercial advertisements. A health-food company marketed a dietary supplement called Vitamin O in full-page newspaper ads. Vitamin O turned out to be ordinary saltwater.