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.
Mona Lisa (Prado's version)
The Prado "Mona Lisa" is a painting of the same subject as Leonardo da Vinci's famous "Mona Lisa" (which is held by the Louvre Museum, Paris). The painting has been displayed in the Museo del Prado in Madrid (Spain) since 1819, but was considered for decades a worthless copy. However, after its restoration in 2012, the Prado's "Mona Lisa" was claimed to be the earliest replica of Leonardo's masterpiece.