"He told me that a very wealthy client rang to tell him that she had just bought a Picasso," says Anderson. "This was in the 1940s and the amount - £10,000 - was a fortune. There was just one problem: Picasso had forgotten to sign the painting. So the client rang Knollys and asked him if, the next time he saw his friend Picasso, he would make sure he signed the painting
Knollys agreed, and his client delivered the painting to Storran gallery. Picasso was in town. Knollys explained the problem. Picasso said that of course he would rectify the omission, but upon looking at the painting said that it was not by him... Picasso then asked him: "How good a client is the owner?" Knollys said that the lady in question was one of his best clients. "In that case," said Picasso, "the painting is by me." Anderson says: "he signed it and the painting has gone on to be authenticated as a genuine Picasso. But is wasn't by him."

Alex Wade.  Master Criminals. The Guardian. G2. page 16.  24/05/05.

Peculiar, that our modern culture of the copy should opt... for the authority of the signature, since no two signatures are exactly the same.

... signatures acquired their full authority only with the Romantic celebration of genius, when devotees of the autograph snipped signatures from letters and mounted them beneath engraved portraits as if the hand underwrote the face. 

Schwartz, Hillel. The culture of the copy: striking likenesses, unreasonable facsimilies. Zone books. 1996.