The Haskell Free Library and Opera House (French: Bibliothèque et salle d'opéra Haskell) is a neoclassical building that straddles the international border in Rock Island (now part of Stanstead), Quebec and Derby Line, Vermont. The Opera House opened on June 7, 1904, and was deliberately built on the border between Canada and the United States. It was declared a heritage building by both countries in the 1970s.
Today, the library has two different entrances (one from each country), and hence, two different addresses: its American address is 93 Caswell Avenue, while its Canadian address is 1, rue Church (Church Street). Exiting the library through the opposite entrance requires one to report to the country's customs thereafter.
More information on the Wikipedia page