Donation of Constantine
The Donation of Constantine was a letter thought to be written by Roman Emperor Constantine, gifting land (parts of the western Roman Empire, which today would be northern Italy) to Pope Sylvester I for curing his leprosy and for converting him to Christianity.
As it turns out, the lands acquired in 756 by the papacy were more likely due to Pope Stephen II's negotiations with Pepin the Short and political games between the church and the Frankish Empire than any alleged donation.
The Donation of Constantine was revealed as a forgery in 1440, after Lorenzo Valla pointed out several factual errors in the document, including references to temples that did not yet exist when the document was allegedly drafted -- and the problem that Constantine never suffered from leprosy. The Catholic church recognized the forgery and returned the land to Italy in 1929.