As the carpenters say, "Measure twice..."

The Blunder of Fort Blunder
While nowadays, a war between Canada and the United States is laughable, in the early 19th century preparing for just such a scenario was the top priority to a great many military planners. During the Revolutionary War the Continental Army attempted to invade that other British colony in North America, using colonial militia, with terrible results. At the beginning of the War of 1812 another attempt was made again, with equally disastrous results. The Canadians and British retaliated in 1814 and also failed. So, while relations between the US and Canada today are quite amicable, back then they were a bit strained.
One fear on the part of the Americans was that the British or Canadians could use Lake Champlain as an invasion route into New York. This was not unreasonable, as both sides had used the lake as a corridor for invasion since the French and Indian War. In 1816, the US Government decided that it was necessary to build a fort along the lake to fend off a hypothetical invasion. A sandy island called Island Point was chosen as an ideal strategic location as it jutted out into Lake Champlain.
Construction was a haphazard affair as the sandy island provided a poor foundation for stone fortifications. Despite the challenges construction carried on. The fort was even visited by President James Monroe in 1817 in an effort to encourage the workforce. Building of the fort was based upon an old survey done decades before however, after two years of construction, the contractors decided to re-survey the area. It was then that they discovered something very disheartening; the fort had been accidentally built on the wrong side of the border, on Canadian soil. Construction of the fort was immediately halted.
The building project was later jokingly named “Fort Blunder”. Over the coming years the stone and timber of the fort was scavenged by local Canadians to build houses, towns, and churches. After a few decades nothing was left of Fort Blunder. The picture above is of a similar fort called Fort Montgomery, which was constructed in 1842 after a treaty readjusted the border, ceding Island Point to the United States.