Kaskaskia, Illnois

Most people think the Mississippi River separates Illinois from Missouri, and indeed Kaskaskia Illinois has a Missouri zip code. But it's actually a small anomaly in the border that jumps across the Mississippi River, due the the movement of the river itself. Read about this little town / area, it's quite interesting...

From the Wikipedia:

Kaskaskia is a village in Randolph County, Illinois, United States. In the 2000 census the population was 9, making it the smallest incorporated community in the State of Illinois in terms of population. A major French colonial town of the Illinois Country, its peak population was about 7,000, when it was a regional center. Kaskaskia was the capital of the Illinois Territory, created on February 3, 1809, and in 1818, when Illinois became the 21st U.S. state, it briefly served as the state's first capital until 1819, when Vandalia became state capital of Illinois for the following twenty years, 1819 to 1839.

Most of the town was destroyed in April 1881 by flooding, as the Mississippi River shifted eastward to a new channel, taking over the lower 10 miles of the Kaskaskia River. These were the results of deforestation of the river banks during the 19th century, due to the need for wood fuel to feed the steamboat traffic. The river now passes east rather than west of the town. The state boundary line, however, remained in its original location. Kaskaskia, essentially an island, is one of the few portions of Illinois west of the Mississippi and can only be reached from Missouri. A bridge crosses the old riverbed, a creek or bayou that periodically is full of water.