From the voyages of Maelduin and St. Brendan to the Crocker Land Expedition, islands observed then lost to later travellers persist. While sailors are cast as unreliable witnesses and prone to tall tales, phantom islands persist in adventure stories even where the world is mostly known. In some cases it may be a simple case of mistaken navigation, where a fog bank or sand bar is mistaken for something more substantial.
Other circumstances have fantastic explanations like the leviathans of old; the Old English Fastitocalon mentioned by Tolkien in Tom Bombadil or the zaratan catalogued in Borges' Book of Imaginary Beings and later in al-Qadim. Others may be wreathed in enchanted mist like Hy-Brasil, set to appear when the stars are right. Though that phrase may indicate a seagoing encounter with a more horrific outcome of course.
All are opportunities for adventures. Misplaced navigation may take an expedition into a new world. Yet other errors are introduced by map makers, intended to flatter the courtly patrons of expeditions or deceive the gullible. When these maps hold promises of wealth or new lands, adventurers are drawn like moths to flame. And in many of the worlds we explore, there be dragons.