Valdemar II
Banned
Since everyone ignored this, I'll point out that that's a commonly held belief, which is wrong, IIRC. QI said so. I'm not sure if they thought it was a globe, but they definitely did not think it was flat.
http://en.wikipedia.org/wiki/Flat_Earth
Wiki even agrees.
Christopher Columbus sailing west in order to get to the east years before whatshisname (I've forgotten. Pretty sure it wasn't Galileo) suggested that the world was, indeed, round does rather suggest that it is a fallacy that the flat Earth was a widely held belief.
It was never a common held belief, but there was a lively debate in Europe in the 12th century, where theologians argued the common belief that the Earth was round was superstitious nonsense (they based their argument on the Bible), but the discussion ended with the traditionalist side winning , and the fact that the Earth was round was commonly known among learned men and sailors at Columbus time just as it had been a millenium earlier.