A case of divine intervention:
The Last Supper by Leonardo da Vinci is not a painting in a museum, but it's painted in the north wall of a Cloister in the Church Santa Marie delle Grazie (Church of Holy Mary of Grace) in Milan, Italy. This church was bombed by the Anglo-American Allied forces during WWII. From
Miracolo a Milano: il salvataggio dell'Ultima Cena di Leonardo :
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On the night of August 15-16, 1943, Leonardo da Vinci's Last Supper risked disappearing forever when a bomb fell in the Cloister of the Dead, causing the collapse of the east wall of the refectory housing the work:
the bomb missed the masterpiece by only 20 meters. The north wall, which houses the Last Supper, had been reinforced by Italian museum officials in 1940 with sandbags, wooden scaffolding and metal reinforcements, which prevented the wall from collapsing.
As luck would have it, no bomb fragments reached the north wall, igniting the sacks or wooden structures."
After the bombardment, you can see the North wall from the back still standing
"Basilica of Santa Maria delle Grazie, in Milan, gutted by the bombing: visible are the wooden scaffolding, sandbags, and metal reinforcements, on both sides of the northern wall of the refectory of the convent attached to the basilica, put up to protect the Da Vinci Last Supper that safeguarded it"
