The border between Belgium and the Netherlands at Baarle-Hertog is one of the messiest in the world. It includes bits of Belgium in the Netherlands, and bits of the Netherlands in the bits of Belgium that are in the Netherlands.
The Treaty of London, in 1839, represented official international recognition of the independent Kingdom of Belgium. But it did not fully settle the border between Belgium and its norther neighbour the Netherlands… because that border was hopelessly complicated. Various Medieval lords had traded land back and forth for centuries, and those divisions carried over into 19th century negotiations.
After four more years of negotiation, the 1843 Treaty of Maastricht finally formalised the border. And the result in one town (or two towns?) was an absolute mess:
Baarle-Hertog is a Belgian town just north of the border. It’s a series of enclaves, islands of Belgium in the midst of the Netherlands. Surrounding and intermingling with Baarle-Hertog is Baarle-Nassau, the Dutch town. The borders cross streets and bisect houses; there are pockets of one country as small as 2500 square metres. And it gets even messier.
Within the tiny Belgian enclaves are some even smaller pockets that remained part of the Netherlands. Formally these are known as counter-enclaves, or enclaves within enclaves – and almost all of the world’s supply exists right here. So you might live in a house that is part of the Netherlands, walk out the front door and be in Belgium, and then cross the road to re-enter the Netherlands.
As you can imagine, the two countries’ differing approaches to Covid-19 restrictions have made things especially complicated. Say Belgium closed down shops but the Netherlands kept them open. What would happen to the local Zeeman clothing store, which crosses the border?
(Apparently they closed the Belgian half and kept the Dutch half open.)
The Generalist Academy 
