As a rely to your broad question, yes. There are various instances where people doubted the claims of explorers. The most famous instance that comes to mind is the argument between John Hanning Speke and Richard Burton over whether or not Speke had truly discovered the source of the River Nile (he had). They had gone on a joint expedition together but Burton fell ill (they had both been ravaged by malaria during various points of the trip) and stayed behind at a local village while Speke pushed on to arrive at what would become Lake Victoria. After arriving back in London, Burton started a smear campaign against Speke and it even led to the Royal Geographical Society requesting a public debate between the two to present their evidence and settle the matter once and for all. However, in a bizarre twist of events, Speke died the day before due to a hunting accident, when his gun accidently discharged in his hand while he was climbing a small wall on the property he was hunting on, dealing him a fatal wound. The animosity was so strong between Burton and Speke that Burton suggested it was suicide, so that Speke would not have to face Burton in a public debate. For sources, I would suggest "Explorers of the Nile" by Tim Jeal. It is a fantastic book if you are into that sort of thing.

For another example, Robert E Peary claimed to be the first to reach the North Pole. However, many modern historians and explorers doubt the claim, but it is unclear if Peary purposely lied about his achievement or if he truly believed he had reached the North Pole. Source here.

So, I am not a historian, but I've been recently reading "The Travels of Marco Polo", and I think a few excerpts from the introduction by Milton Rugoff will be informative here. The tl;dr version is that basically people just straight up did not believe Marco Polo, and thought he was making it all up, to the point that he was asked by friends to recant on his deathbed. It should be noted that modern scholars and explorers have found his reports to be, for the most part, accurate:

"Marco Polo's book was for all practical purposes not only the first to pull aside the veil that hid the East, but it was nearly the last for a long time to come. A few men followed in his steps during the early decades of the 14th century: Ibn Battuta, a North African Moslem who is said to have covered 75,000 miles during almost thirty years of roving from Spain to Carthay; and Frian Odoric, a saintly ascetic from the Udine area, who also reached the East. But we find ourselves turning from their journals, valuable as they may be, with a renewed respect for Marco Polo's scope, penetration, and raging curiosity.

"Marco's account should thus have burst upon his contemporaries as a kind of revelation. It did nothing of the kind. The Europeans of the 14th and 15th centuries, still children of the Middle Ages, were not particularly influenced or impressed. The majority, even among the literate, continued to be content with fragments of fable or fantasy. Even those who should have welcomed his report, the map makers and geographers, seem in general to have looked on his book as more romance than fact.

Far from being a science, mapmaking was still an exercise of fancy, or religious devotion, or fear of the unknown. In the early Christian era - by which time such works as Ptolemy's atlas and the reports of Herodotus had been forgotten or disdained - a leading geographer of the early 5th century, Cosmas, thought that the universe was a flat parallologram and the earth a dome-shaped mountain with Jerusalem as center and summit. And as late as the 14th century another renowned geographer, Cardinal d'Ailly, concluded that Africa must not be far from India because both contained elephants."

Rugoff also states,

"Still another very old tradition says that few who heard Messer Marco's stories took them literally and that he came to be called 'Marco Milione' and the house he lived in 'Corte di Milioni' - both names usually assumed to be jibes at the extravagant numbers in his stories."

Rugoff also notes,

"We have, in fact, come a long way from the days when, if we may believe certain traditional stories, Marco Polo's book was a synonym for extravagant claims and tall tales. We are now much more likely to believe the story that when friends urged him in his last hours to modify or retract some of his statements, he said: I did not write half the things I saw."

By the way, as to the accuracy of Marco Polo's accounts:

"As for Marco's accuracy and credibility, the studies of many scholars and the investigations of explorers such as Sven Hedin and Aurel Stein have gradually confirmed these to the point where only a handful of passages seem irredeemably gullible or superstitious. Among these are the legend of a cobbler whose faith moved a mountain; the roc, a bird that could carry off an elephant; dog-headed men in one place and dog-tailed men in another; an Island of Females inhabited by 'Amazons'; and the tomb of Adam on Ceylon. But in all fairness it must be added that most of these are plainly secondhand and not vouched for."

All of these quotes are by Milton Rugoff, in The Travels of Marco Polo Signet Classics edition, copyright 1961.