From The Art Newspaper:
Although the museum had known since the 1970s that there was more to The Last Drop (The Gay Cavalier) (around 1639), by the Dutch artist Judith Leyster, its conservators only recently were able to remove overpaint, revealing a skeleton that Leyster had included as a moralising tale of what happens to those who drink to excess. “We didn’t know enough about the painting’s condition to determine if the overpaint could be removed without causing any damage,” says the museum’s director of conservation Mark Tucker. He suspects that the skeleton was removed before Johnson bought it in 1908 to make it more marketable.
More about the painting and the artist, Judith Leyster, at the wiki link.