It's much easier to tell a story about a trip or an expedition that you didn't take than to actually make the trip. But the tallest tales are figured out to be hoaxes eventually. For example:
Frederick Cook almost certainly set foot in many places where previously no person had before—but the New York-born explorer is also seen as one of modern exploration’s most notorious fraudsters. He participated in three significant expeditions between 1891 and 1903, two of them into the Arctic and the latter a circumnavigation of Alaska’s Mount McKinley, also known as Denali. In 1906, he set forth on another McKinley outing, this time returning home to report that he had summited the 20,320-foot peak, which had never been climbed before. The claim stood the test of time for only three years, when the true story came spilling out: Cook had taken his summit photo on a tiny mountain 19 miles from McKinley’s peak.
Suspicions about Cook's Mount McKinley climb escalated after his claim to have reached the North Pole in 1909 was disputed. But that's another story in the list. The nine stories at Smithsonian include swimming, racing, sailing, and several mountain summits. Link
What about the moon landing?
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