Today's date (Dec 4) 1961, Henri Matisse’s artwork called Le Bateau was finally flipped around to be displayed right-side-up at the Museum of Modern Art in New York.
It had been hung upside down for around 46 or 47 days! Stock-broker Genevieve Habert noticed it was displayed upside down. Something like 116,000 people visited the museum to see the artwork before anyone noticed it was upside down. Even Matisse's son, Pierre Matisse, had seen the exhibit and didn't notice anything wrong.
According to TIME MAG, people didn't believe Mrs. Habert at first... but she "stuck to her guns" and eventually she called the New York Times. After that, the museum finally listened. They were like "Oh hey, would ya look at that, it IS upside down!"
