In 2007 Peter Silverman bought a portrait titled Young Girl in Profile in Renaissance Dress for $19,000. Christie's had previously sold it in 1998. At that sale they described it as a 19th Century German drawing.
Silverman thought there was more to the portrait than met the eye, so he decided to have it examined further. The Paris lab he used to assess it found a fingerprint on it that they said substantially matched a known finger print of Leonardo da Vinci.
The drawing was done by a left-handed person and stylistic mannerisms, carbon dating and the dress the young girl wears in the drawing match 15th Milanese fashion.
If verified as a da Vinci the drawing would be worth tens of million of dollars.
Aside from wondering exactly how a da Vinci gets lost, I feel sorry for the poor person who sold such a valuable object. I wonder if they got tired of it hanging in their living room and replaced it with an Alma Thomas masterpiece? Regardless, finding out you peddled a da Vinci it must feel make you feel kind of like you're the Fifth Beatle.