Cartographic vandalism is actually quite rare. Most of the errors on maps are honest errors, but because of the way that cartographers would borrow from other maps, one cartographer's error can end up in other maps for centuries. One famous example is California as an island; maps from the 16th, 17th, and 18th centuries frequently depicted California and the Baja Peninsula as an island. Gerardus Mercator got it right with his world map of 1569, yet maps depicting California as an island became the norm through much of the 1600s. The problem was so controversial that eventually Ferdinand VII of Spain stepped in and decreed that it was not an island.
Mercator might have gotten California right, but he was way off regarding another island. Early cartographers needed to look everywhere for their sources, and when mapping the North Atlantic, Mercator turned to Inventio Fortuna, an account of a Franciscan monk's travels in the North Atlantic. Or rather, he turned to second-hand accounts, as the original manuscript had been lost a century earlier, and all that remained were secondhand accounts of it. As Mercator describes it:
"...In the midst of the four countries is a Whirlpool into which there empty these four Indrawing Seas which divide the North. And the water rushes round and descends into the earth just as if one were pouring it through a filter funnel. It is 4 degrees wide on every side of the Pole, that is to say eight degrees altogther. Except that right under the Pole there lies a bare rock in the midst of the Sea. Its circumference is almost 33 French miles, and it is all of magnetic stone. And is as high as the clouds, so the Priest said, who had received the astrolabe from this Minorite in exchange for a Testament. And the Minorite himself had heard that one can see all round it from the Sea, and that it is black and glistening. And nothing grows thereon, for there is not so much as a handful of soil on it."
Part of the reason these errors are so fascinating is that it's entertaining to imagine a world just subtely different from our own. Jules Verne thought so too, and in his novel The Adventures of Captain Hatteres, he tells the story of a British explorer who travels to the north pole and encounters there a vast polar sea, and at the center of it a volcano, much like the Rupes Nigra that appeared on earlier maps.