He later quietly and privately backed off of Dianetics, seeing that it was spawning a cult religion. Campbell was the champion for many scientific theories and the most famous besides Dianetics, which turned our more negative than positive, was the Cleve Cartmill affair(from Answers.com):
Cleve Cartmill (1908 " 1964) was an American author who specialized in writing science fiction short stories. He is best remembered for what is sometimes referred to as "the Cleve Cartmill affair", when his 1944 story Deadline attracted the attention of the FBI due to its detailed description of a nuclear weapon similar to that being developed by the highly classified Manhattan Project.
Before embarking on his career as a writer for pulp magazines, Cartmill had a number of jobs including newspaperman, radio operator and accountant, as well as a short spell at the American Radium Products Company. Many of his earliest stories, from 1941 onwards, were published in John W. Campbell's magazines Unknown and Astounding Science Fiction. This was at the start of World War II, when Campbell found himself short of material because many of his regular writers were away on military service (from which Cartmill was exempt for medical reasons).
In 1943, Cartmill suggested to Campbell that he could write a story about a futuristic super-bomb. Campbell liked the idea and supplied Cartmill with considerable background information, gleaned from unclassified scientific journals, on the use of Uranium-235 to make a nuclear fission device. The resulting story, "Deadline", appeared in the issue of Astounding dated March 1944, which actually appeared early in February of that year. By March 8th it had come to the attention of the Counter-Intelligence Corps, who saw many similarities between the technical details in the story and the research currently being undertaken in great secrecy at Los Alamos. Fearing a security breach, the FBI began an investigation into Cartmill, Campbell and some of their acquaintances (including Isaac Asimov and Robert A. Heinlein). It appears that the authorities eventually accepted the explanation that all the story's material had been gleaned from unclassified sources, but as a precautionary measure they requested that Campbell should not publish any further stories about nuclear technology for the remainder of the war.
Historical interest aside, "Deadline" is not one of Cartmill's best stories, being described by Robert Silverberg as "a klutzy clunker" and by Cartmill himself as "that stinker". According to Silverberg, Cartmill also used the phrase "it stinks" when describing the story to a postman who was acting as an informer for military intelligence.
Apart from the "Deadline" incident, Cartmill's writing career was undistinguished but competent. In his book A Requiem for Astounding, Alva Rogers expresses the opinion that "Cartmill wrote with an easy and colloquial fluidity that made his stories eminently readable". Outside his writing career Cartmill was likely best known, at the time, for being the co-inventor of the Blackmill system of high speed typography.
His son, Matt Cartmill, is a Professor of Biological Anthropology at Boston University and a science writer to whom Robert A Heinlein partly dedicated his 1947 book Rocketship Galileo.