A good example of this is what is now known as the Piltdown fraud, in which some fossil experts found some modern human remains, yet they altered the bones to make them appear to be “the missing link.”
The fraud was constructed by careful altering of a human skull and an ape's jaws. The bones were altered by the hoaxer or hoaxers (whose identity, despite various theories, is still unknown). There was no altering of the finds by any scientist who considered them genuine "finds". I happen to suspect that the discoverer, Charles Dawson, was the hoaxer. Although the principal experts that became involved with the find -- Sir Arthur Smith Woodward, Grafton Elliot Smith, and Sir Arthur Keith -- have been accused of participation in the hoax at various times, but not (IMHO) convincingly, and in any case were not suspected by contemporaries.
IOW if you're meaning to suggest, as you seem to be, that scientists "altered" some kind of an otherwise genuine find, this is unquestionably false. This was no opportunistic and after-the-fact alteration. It was a purposefully excuted and premeditated fraud, executed with materials not found in the ground, but rather acquired specifically for that purpose. Much as may not be known, that much is certain.
The skull was from a human and the jaw was from an ape. For forty years this forgery was hailed as the “missing link” until it was exposed as forgery in 1953.
It is not remotely accurate to claim that "this forgery was hailed as the 'missing link'" for the whole of that forty years.
In the first place the initial and general reaction of the anthropological community -- aside from Smith, Woodward (who was not actually an anthropologist), Keith and a few other enthusiastic English scientists -- was strong and sometimes vociferous SKEPTICISM. This was true at least from the first announcement of Piltdown in 1912, to the announcement of the Piltdown II find in 1917. Most experts, particularly French, but also leading Americans, argued that the skull and the jaw obviously came from different creatures, and that the association was fortitous, i.e. coincidental.
This of course was correct, and critics also correctly identified the skull as human and the jaw as simian. The only problem was that no one seriously mooted the idea of fraud, and so most (but not all) of the critics were silenced by the Piltdown II find. Once coincidence of a human skull and an ape jaw ending up together might be admitted, but two separate such associations couldn't be creditably attributed to chance.
This web page summarizes some of the opposition to Piltdown, the linked section debunking the myth that "The hoax was swallowed uncritically":
This is a half truth; almost no one publicly raised the possibility of a deliberate hoax. There were rumors circulating, however. William Gregory, a paleontologist at the American Museum of Natural History wrote in Natural History in May of 1914:
"It has been suspected by some that geologically [the bones] are not that old at all; that they may even represent a deliberate hoax, a negro or Australian skull and a broken ape jaw, artificially fossilized and planted in the grave bed, to fool scientists."
He went on, however, to vigorously deny the charge, concluding
"None of the experts who have scrutinized the specimens and the gravel pit and its surroundings has doubted the genuineness of the discovery."
In general, however, the finds were accepted as being genuine fossils but were not accepted uncritically as being from an ancient human ancestor. There was an early and recurring doubt that the jaw and the skull were from two different animals, that the jaw was from an archaic chimpanzee and that the skull was from a relatively modern human being. Notable critics include Dr. David Waterston of King's College, the French paleontologists Marcellin Boule and Ernest Robert Lenoir, Gerrit Miller, curator of mammals at the Smithsonian, and Professor Ales Hrdlicka.
Initially there were many more critics, e.g. Osborn. However the finding of the second skull converted many of the critics. Finding a jaw from one animal near the skull of another might be an accident of juxtaposition -- two such finds is quite unlikely to be an accident. Some critics, e.g. Lenoir and Hrdlicka remained unconvinced none-the-less.
The following quote comes from a "The Evolution of Man", a 1927 book by Grafton Elliot Smith:
"Yet it [the skullcap] was found in association with the fragment of a jaw presenting so close a resemblance to the type hitherto known only in Apes that for more than twelve years many competent biologists have been claiming it to be the remains of a Chimpanzee."
Franz Weidenreich in 1946, in his book "Apes, Giants, and Men" (Note that Weidenreich was an extremely respected scientist, having done most of the work on the Peking Man skulls):
In this connection, another fact should be considered. We know of a lower jaw from the Lower Pleistocene of southern England which is anatomically, without any doubt, the jaw of an anthropoid. The trouble is that this jaw, although generally acknowledged as a simian jaw, has been attributed to man because it was found mixed with fragments of an undoubtedly human brain case. I am referring to the famous Piltdown finds and to Eoanthropus, as the reconstructed human type has been called by the English authors... Therefore, both skeletal elements cannot belong to the same skull.
It should also be mentioned that in 1950 Ashley Montagu and Alvan T. Marston mounted major attacks on the interpretation of the Piltdown fossils as being from a single animal.
In addition to what you posted, Friedrichs and Weidenreich had both, by about 1932, published their research suggesting the lower jaws and molars were that of an orang. They were correct.
Only the creationists have failed to get the message. Of course, they don't tend to read the technical literature, and don't care if their science is correct. ANYTHING to defeat those nasty evilutionists!