A gouache of a sailboat and its reflection, by Henri Matisse, that was hung for 47 days, from Oct. 18 to Dec 4, 1961, upside down at the Museum of Modern Art in New York City. 116,000 visitors failed to notice, including Pierre Matisse, the artist’s son. Genevieve Habert, a Wall Street stockbroker, figured out that something was wrong and was eventually able to straighten the painting out.