What happens when a hoaxer owns up and nobody believes him? Dan Gardner's new book "Future Babble" explains the many ways in which experts refuse to admit that they were wrong about something. They ignore contrary evidence, point selectively to facts that weigh in their favor or simply shrug off the mistake as a problem of timing. Even cultists redouble their faith when the world does not end as forecast. These biases are also apparent in the astonishing determination of many people to dismiss the claims of confessed hoaxers.

Mr. Gardner tells how, in the aftermath of 9/11, a seemingly spooky Nostradamus prophecy went viral on the Internet: "In the City of God there will be a great thunder, two brothers torn apart by Chaos, while the fortress endures, the great leader will succumb.'' In vain did a Canadian student confess that he had written it a few years before to mock Nostradamus' vagueness.

Something similar happened with a speech supposedly given by Chief Seattle in 1854 upon signing a treaty surrendering his tribe's lands: "How can you buy or sell the sky...man did not weave the web of life—he is merely a strand in it. Whatever he does to the web, he does to himself.''

In vain do scholars point out that this hymn to ecology was written for television in 1971 by a Texan screenwriter named Ted Perry, who has repeatedly tried to set the record straight. Even Al Gore, in his book "Earth in the Balance," quotes the whole speech, though he nods toward the controversy by suggesting—rather limply—that "the power of his response has survived numerous translations and retellings."

In the early 1920s, Arthur Conan Doyle, the creator of Sherlock Holmes and a convinced spiritualist, befriended Harry Houdini, a determined skeptic. Houdini's problem was that in playing ingenious tricks to persuade Conan Doyle that spiritualists were tricksters, he only succeeded in reinforcing Conan Doyle's view that the magician had spiritual powers.

In one incident, Houdini had a cork ball dipped in white ink spell out a message on a slate suspended from wires. The message was the exact phrase that Conan Doyle had written on a scrap of paper a few minutes before, three blocks away. Houdini had swapped the cork ball for a ball with an iron core, swapped the message for a blank sheet of paper (under the pretense of ensuring that it was folded) and communicated its contents subtly to an assistant behind the wall, manipulating a magnet.

Something similar happened in 1991 when Doug Bower and Dave Chorley confessed to having invented crop circles—symmetrical patterns of flattened wheat that began appearing in southwestern Britain in the 1980s—after an evening at a pub. By the time they went public, crop circles had become a huge phenomenon, pored over by enthusiastic believers who called themselves "cerealogists" and attributed the patterns to UFOs, ball lightning, plasma vortices and the like.

Many of these self-styled experts were making a good living on book sales and were not about to let Doug and Dave rain on their parade. So, with one or two creditable exceptions, they insisted that the men's confession was itself a hoax and pointed credulous reporters to evidence that all the crop circles might not be man-made (too many circles, no witnesses, no footprints, etc.)

It was the incidental details that made the story of Messrs. Bower and Chorley believable. Mr. Bower's wife had grown suspicious about the high mileage on the family car, and most early crop circles appeared on Friday nights—pub night. Their technique was "plank-walking": tying a rope to both ends of a plank of wood and using it as a sort of treadle to push down corn as they walked.

The moral of the tale for magicians, practical jokers and screenwriters is that if you perform your trick too well, it may take on a life of its own and escape into the wild.