Isleworth Mona Lisa
According to Leonardo's early biographer Giorgio Vasari, Leonardo had started to paint "Mona Lisa" in 1503, but "left it unfinished". However, a fully finished painting of a "certain Florentine lady" surfaced again in 1517, shortly before Leonardo's death and in his private possession. The latter painting almost certainly is the one that now hangs in the Louvre. Based on this contradiction, supporters of the authenticity of the "Isleworth Mona Lisa" claim it is the unfinished "Mona Lisa", made at least partially by Leonardo, and the Louvre "Mona Lisa" a later version of it, made by Leonardo for his own use.
Yves Chaudron
Decker asserts that six Chaudron copies had already been sent to the United States ahead of the theft and while the stolen original remained in France, Valfierno followed his fakes and sold each for up to 300,000. The original remained hidden for two years until Peruggia, presenting himself as Leonardo Vincenzo, tried to sell it in Florence, Italy. A museum administrator there was suspicious, reported the attempted sale and Peruggia was arrested. The original "Mona Lisa" was returned to the Louvre in 1913. 
More than 100 years later, none of these six paintings has ever surfaced, bringing into question if they ever in fact existed. Further, the only evidence that the "famed forger" Yves Chaudron even existed is Karl Decker's 1932 "Saturday Evening Post" article. There are no known photos of the man or relatives or any biographical details outside of this incident. None of his other forged works has ever come to light, and it is quite impossible to be a 'famed forger' if none of your forgeries is discovered. Decker's entire 1932 article may be a work of fiction that, much like forgeries in the art world, has fooled many who have read it.