The Haydn expert H. C. Robbins Landon recently hailed the discovery of Haydn's six lost sonatas as "the greatest musicological coup of the century." The haste of his retreat resembled that of the historian and Hitler expert Hugh Trevor-Roper, who proclaimed as genuine a purported trove of Hitler's diaries that proved to be blatant forgeries.

When musicologists or historians stumble, they stub only their own toes. But when scientists err, there's usually more than an individual's faulty judgment to blame. Scientific claims, after all, owe their authority to being the certified intellectual product of a community of experts. In principle, the maker of a scientific discovery has tested his idea by rigorous experiment, and his professional peers have rigorously checked it for flaws before publication.

With such guarantees, can scientists' announcements of discovery ever be wrong? They sure can. As three recent episodes show, even the most important scientific claims can come embarrassingly unglued.

Last July, a group of Danish and other scientists declared the earth's climate was inherently unstable, implying that the planet might plunge without warning into a new ice age. Their report had credence because it was based not a computer's projections but on real evidence, the amazing natural archive of climatic data held in the annual layers of snow packed into the Greenland ice cap.

Drilling down 10,000 feet, the Danish scientists reached layers of ice formed by snow that fell before the end of the second most recent ice age. This bottommost stretch of the core spanned an epoch of the utmost interest -- the 20,000-year period, analogous to our own, that separated two ice ages.

The news from this interglacial period, the scientists reported, was chilling. The earth's climate then, far from being serenely stable as it has been since the end of the last ice age 10,000 years ago, blew hot and cold in sudden, violent shifts as brief as a decade. Under conditions like this, extensive agriculture, even life in general, would be difficult.

Climate experts seized on the new data to repeat their warnings against disrupting the present climate with industrial waste gases. "If the Earth had an operating manual," noted a stern commentary on the new report in Nature, "the chapter on climate might begin . . . don't touch the dials."

Happily, in this case, the collective experts had seriously misread the operating manual. In December, a team led by American scientists reported on a second deep ice core drilled about 17 miles away from the first. The two sets of climate data matched perfectly except in the interglacial period. The explanation turned out to be simple. Where the ice at the bottom of the vast Greenland glacier flows over the rough bedrock, it churns up in waves that fold and mix the neatly graded layers of snow. The data for the interglacial period have been mixed up in this way and cannot yet be interpreted. The Danish team had been seduced by the beautiful regularity of the top 90 percent of its core into assuming it went all the way down.

Another important discovery to have hit the rocks in recent months was the claim of scientists at the Harvard Medical School and Massachusetts General Hospital to have developed a new drug treatment for AIDS. The therapy "may be the Achilles' heel" of the AIDS virus, they declared. It was based on a novel idea by a medical student in defiance of conventional wisdom. Since the virus develops resistance to drugs like AZT and DDI, the student's concept was to use three such drugs together, on the theory that the virus would inactivate itself in the effort to resist all three at once.

In test tube experiments, the triple drug therapy seemed to work. The virus developed the three expected resistance mutations and became inactive. But after publication, when the student's senior colleagues checked his results more carefully, they found unnoticed extra mutations. It was these that had inactivated the virus. "We are embarrassed by the unrecognized error," the group's leader, Dr. Martin S. Hirsch, said last July. "I wish we had never used the term Achilles' heel."

A misadventure of a different kind was contributed by astronomers in search of planets around other stars. The race was apparently won in July 1991 by Andrew G. Lyne, a radio astronomer at the University of Manchester, in England. Blips in the radio signals from a star, he and colleagues said, showed it must be encircled by a planet about 10 times the mass of Earth.

But planet Phoenix, as it was named by other experts, soon turned back to ashes. Six months later, Dr. Lyne, to his credit, recognized and published an error in his calculations: he had failed to allow completely for the fact discovered by Kepler in 1609 that the earth's orbit around the sun is not circular but slightly elliptical. With this adjustment, the evidence for the inferred planet evaporated. Presumably elated at finding the distant planets that all astronomers believe must exist, his colleagues and the referees who read his paper before publication also overlooked the blunder.

When other experts goof, it's an individual error of judgment. When scientists get things wrong, it's legitimate to ask what went wrong with the vaunted checking procedures on which the credibility of science rests.

Science is a hard calling. Without the zeal to understand nature, few would endure the tedium of the laboratory. But that same enthusiasm makes scientists strong believers in their own ideas, despite graduate school bromides about objectivity. Unfortunately no amount of checking can save a scientist from sometimes deceiving himself. Then, when an idea is particularly plausible or appealing, his hapless colleagues may all be lured into the same trap.

Such cases are rare but striking. Each of the three above concerned a major discovery announced by leading scientists and published in Nature, a prominent scientific journal. True, each was fairly quickly corrected. But the experiences suggest a principle to apply to scientific claims made by even the most eminent institutions: Don't fully believe a word until at least six months after publication.