What was wrong with the line drawn at Tordesillas?

Nobody could find it. The 1494 treaty set the boundary at 370 leagues west of the Cape Verde islands, and measuring longitude at sea was unsolved for another two and a half centuries - so neither party could establish where the line actually fell, and both interpreted it favourably. It also did not specify which league.

The consequences were large and largely accidental. The eastern bulge of South America fell on Portugal's side, which is why Brazil speaks Portuguese - a language boundary set by a line nobody could locate, six years before any European reached the coast. When the two powers met again on the other side of the world in the Moluccas, the argument had to be settled by a separate treaty and a cash payment.

The treaty also assumed the Pope could allocate the planet between two Iberian kingdoms, which the other European powers eventually declined to accept. Francis I of France is supposed to have asked to be shown the clause in Adam's will that divided the world without him - and English, Dutch and French expansion proceeded on the principle that a papal bull was not a property deed.