Steve Martin responds in the New York Times to the news that he had owned a faked painting citing the promise of the gallery who sold it to him that they would bear any responsibility if it was determined that Martin was in any way liable:

“It wasn’t clear that it was a fake until after Christie’s had sold the picture, it was a long time after that, that it became known,” he said. […] Mr. Martin said he had purchased forged artworks “once or twice in my life” previously, “and each time you become more and more cautious.”

“You always have to guard against it,” Mr. Martin said, adding that in this case, “The fakers were quite clever in that they gave it a long provenance and they faked labels, and it came out of a collection that mingled legitimate pictures with faked pictures.

Forged Painting Was Once in the Collection of Steve Martin, Police Say (New York Times)