You hear about this sort of thing from time to time; here are two recent examples.
The guy in the first picture above is a businessman from England. He was in Las Vegas and dropped by a garage sale. He picked up the drawing he’s holding for $5.00, a picture of a singer and actor from the 30’s named Rudy Vallee. Turns out that the sketch was made by Andy Warhol when he was 10 or 11 years old, and represents what is thought to be the artist’s earliest known example of Pop Art.
In the second picture, the pictured man was walking through a thrift store in Columbus, Ohio, and saw a poster that caught his eye. He bought it for $14.14. He looked on the back and saw a signature in red pen: “Pablo Picasso”. As it turned out, he had purchased what looks like an artists’ proof, a linocut, #6/100, advertising an exhibit of Picasso’s ceramic work in 1958.