Columbus makes a few mistakes....

     The diameter of the spherical Earth had been calculated accurately by the Greek Eratosthenes in the second century BC, and his calculation was still widely known in the time of Columbus.  Though no European foresaw what lay in wait for Columbus, since all thought mistakenly that the  Ocean Sea, empty of land, was much larger than it was, almost all who could read and had looked into the subject understood that Columbus was seriously underestimating the overall size of the Earth.*
     Columbus, basing his calculations on inaccurate assumptions, theorized that the east coast of Asia could be reached by a European ship within a few weeks of its leaving port.  The actual circumference of the Earth is about 40,000 kilometers, whereas Columbus assumed it to be closer to 25,000 kilometers.  Compounding his mistake was his misreading - in a Latin translation - of a renowned ninth-century Persian astronomer, Ahmad ibn Muhammad ibn Kathir al-Faghani, known in the West as Alfraganus.  The Persian's correct measurements were given in Arabic miles, which Columbus assumed to be the same as Roman miles.  In actuality, Roman miles are about 25 percent shorter than Arabic ones.  Had the Ocean not held the Americas and the vast sea been empty of land between Europe and Asia, Columbus and his crew, heading west, would have perished in the deep and never been heard from again.  This had indeed been the fate of several earlier (and well-known) attempts.

-Thomas Cahill,  Heretics and Heroes

*No one who knew anything thought the Earth was flat.  This was an anti-Catholic fable created by a nineteenth-century Frenchman named Jean Antoine Letronne and disseminated widely to English speakers by Washington Irving in his unreliable biography of Columbus.