Europeans first reached Brazil in 1500, but for centuries before that any decent mapmaker knew where Brazil was. It was an island a hundred or so miles west of Ireland... and this cartographic phantom endured as late as the 1800s.
When Argentina invaded Britain's Falkland Islands in 1982, one of the most common reactions back in the UK was to wonder why the Argentinians were invading some random islands somewhere off the coast of Scotland. That reaction, it turns out, is part of a surprisingly long and not especially proud tradition of Europeans taking pieces of South American geography and arbitrarily placing them next to the British Isles.
Of course, cartographers knew nothing of the future vast South American country when they placed the island of Brazil, or Hy-Brasil, on their maps, and in all likelihood the two place names are entirely coincidental. Hy-Brasil was one of many such phantom islands that popped up on maps throughout the centuries of European navigation, usually a result of legends being misrepresented as facts and some faulty reporting by sailors. Because of its large size and close proximity to Ireland, Hy-Brasil was always more myth than mistake, a dumping ground for various pieces of folklore and maritime legend. The Strange Maps section of Big Think has an awesome overview of the island's non-history.
Remarkably, the fake island of Brazil arguably has a longer European history than the real country — while Portuguese explorer Pedro Álvares Cabral claimed to have first made landfall in Brazil in 1500, 512 years ago, Hy-Brasil first appeared on European maps as early as 1325 and expeditions to find the place continued until as late as 1872, a run of at least 547 years as a real fake place. Considering the latter-day confusion over the Falklands, one can only imagine how bewildered 17th and 18th century Brits must have been whenever they heard the Portuguese were going to war in Brazil...