The US accidentally built a fort to repel Canada... in Canada
The War of 1812 was a conflict between the United States and Great Britain, though it could be argued that Canadians and Native Americans were most affected by it. The northern theater of the war, especially near the border of Canada and the Great Lakes, saw some of the most intense fighting — so much so that, after the Treaty of Ghent, President James Madison ordered a heavy fortification be built at the northern end of Lake Champlain to prevent future invasion.
The only problem... was that the fort was built on the wrong side of the border.
The Americans didn't discover the error until two years later, when a surveyor found the fort was being built north of the 45th parallel.
Construction halted on the fort, which earned the moniker "Fort Blunder," until 1842, when the U.S. moved the boundary line north (see the current boundary in the image above) — because pride.
The Webster-Ashburton Treaty resolved a number of border issues between what would one day become Canada and the United States, who promptly began to build another fort — this one named for revolutionary war hero General Richard Montgomery. The fort was built from the same limestone slabs that helped raise the Brooklyn Bridge, and, though it was never fully garrisoned, it was armed and ready for action.
As the United States' relationships with Great Britain and Canada flourished, Fort Montgomery's function dwindled. In 1926, it was auctioned off by the U.S. government and sold to a private bidder.
Today, it's actually for sale... but no one wants it (except for me — I absolutely want it and I am currently looking for a $3 million donation for this cause. The parties there will be epic. Bring your boat and everyone you know...), though in 2009, it was placed on The Preservation League of New York State's list of Seven to Save.