The Nazis used the Leaning Tower of Pisa as an observation post. Allies didn't bomb it because it was so beautiful!

de bene esse: literally, of well-being, morally acceptable but subject to future validation or exception 

The Leaning Tower of Pisa is a famous bell tower in the Italian town of Pisa. As it's name suggests, the tower leans to one side in a perpetual state of falling over.
This lean is due to the fact that the tower was built on an unstable foundation. It used to lean at an angle of just over 5 degrees, but since it has been restored, it now leans at just 3.99 degrees.
In WWII, the Allies found out that the Nazis were using the tower as an observation post. They sent a US Army Sergeant to see if this was actually true. He traveled to the tower and was extremely impressed by it's beauty. So he never ordered a strike in order to preserve this piece of art and history.
(Source)