A friend of Pablo Picasso recounts this story...
I was staying with Picasso in his studio. Every day, dealers would come by to authenticate paintings they were trying to sell. They would ask Picasso if the painting was real or a fake.
A dealer came by one day with a painting, Picasso glanced at it, and without hesitating said, "fake." Later that day, two more were identified as fakes.
The next day, a different dealer came by. Picasso hardly looked up. "Fake!" he bellowed.
After the dealer left, I couldn't help myself. "Picasso, why did you say that painting was a fake? I was here, in this studio, last year when I saw you paint it."
Picasso didn't hesitate. He turned to me and said, "I often paint fakes."