One island switches nationality every six months, and has since 1659

Pheasant Island sits in the middle of the Bidasoa River between France and Spain, and belongs to neither exclusively — it's a condominium, jointly ruled by both. Under a rule added to the 1659 Treaty of the Pyrenees, sovereignty alternates every six months: Spain governs it from February to July, France from August to January. It's believed to be the world's oldest continuously alternating condominium, and the tiny island itself is usually kept uninhabited and closed to visitors.

— Wikipedia contributors, Pheasant Island — Wikipedia entry on Pheasant Island's Franco-Spanish condominium under the 1659 Treaty of the Pyrenees