The Vinland Map
The Vinland Map is an apparently fifteenth-century copy of a thirteenth-century map depicting Vinland. It's authenticity is disputed, and most scholars believe it is a modern map drawn on fifteenth-century parchment. One example of the argument for its authenticity can be found here. The parchment itself has been carbon-dated to c.1434AD but the drawing could have been done much later. The fact that the ink has fallen off the parchment in the time since it first came to light is on element that has convinced some scholars that this is the case. Analysis of this ink has suggested that the ink itself is modern. You can explore the evidence yourself here and draw your own conclusions.
The history of the debate about the map's authenticity highlights the difficulty there can be in determining whether an item is genuine or fake. If it is authentic, it is an important document because it proves that Europeans other than the Scandinavians were aware of America before Columbus sailed westwards in 1492. If it is fake, it has less value to us now. However, not all forgeries are worthless. Historical forgeries, such as the fake Vlfberht swords mentioned in this exhibit are important because they show how valued the name Vlfberht was.