One such thing was the existence of Antarctica. This occasionally leads to theories that ancient people must have actually discovered or even settled Antarctica, perhaps at some time when it was not ice-covered. The more reasonable explanation, IMO, is just that they had some mixed-up geographic knowledge that happened to get something right by random chance, given the sheer amount of bad and mixed-up geographic knowledge medieval maps display.
Medieval cartographers had a lot working against them. They had no reliable way to determine longitude, for example, and there was a strong belief that geography actually reflected theology. The quintessential Medieval map is the T-O map, so called because it looks like a T inside an O:
(Note: North is to the left on these. The convention of putting North at the top of the map probably didn't begin until compasses became widespread.)
Obviously no one was actually trying to navigate with these things. They are meant to show that the divine order of the world reflects the Christian Trinity--one continent for each part of the Trinity, with the world's center at Jerusalem.
There was just one problem with this tidy little package: ancient Greek philosophers had claimed the existence of a fourth, southerly continent, beyond the ocean's stream. This mysterious fourth island was known by a variety of names, such as the Antipodes and Terra Australis Incognita--the Unknown Southern Land.
The whole matter occasioned much debate, and not just because people thought three continents was much better than four. For you see, medieval geographers thought the equator was an impassable band of fiery burning heat. (Any Europeans who'd ventured into the Sahara would be forgiven for thinking that going even further south was a bad idea.) If the equator was impassible, then how could anyone know if there was land down there or not? How could anyone get there or live there? And if people did live down there, how could the evangelists have preached there (the Bible claims that they preached in all corners of the Earth, after all)?
But argue as they might, there it was in ancient Greek philosophy, and they were loathe to completely discard anything in Greek philosophy.
Where did the Greeks get the idea?
I'm not sure, but the most obvious idea is that they had contact with people who had actually sailed south of the equator and discovered land there (note that much of Africa is, in fact, south of the equator.) (If I recall correctly, the Egyptians manned an expedition that actually made it around Africa.) Couple these reports with the belief in a circling ocean stream at the equator, and it's not too hard to see how people might have gotten confused.
After a while, Europeans discovered that you didn't actually die hideously of burning fire if you sailed too far south, but belief that there was another island down there somewhere persisted, even as more and more of the world turned out not to contain it.
With each new discovery of land in the southern hemisphere, map makers thought they'd found the edges of the fabled southern continent. But when even the continent south of Indonesia turned out not to extend over the south pole, cartographers threw up their hands and decided to just name it "Australia" and be done with it, somewhere around 1804-1814.
Antarctica was spotted in 1820.
Well, Brazil got its name in a funny way, too.