No evidence exists that any educated person since (at least) the Classical Period believed the world was flat.
You mean it's not????????????????????????? OH CRAP!!!!:).
*grins*
actually, the acceptance of the "people used to believe the world was flat" myth is one of several I have long found puzzling. Another concerns the belief that in days of old people believed the moon actually changed its shape in its monthly cycle.
I have been able to find evidence countering the former myth (Roman imperial regalia and statuary, later European Regal regalia: cf "orbis terrarum") and was recently given the genesis of the myth itself by an Evo here.
I have so far not found evidence countering the latter, but STRONGLY suspect it exists.