The most complicated international border between two countries in Europe is between the Netherlands and Belgium in Baarle-Nassau
The territories marked in yellow are Belgian territory within the Netherlands. As you can see, there are enclaves within those enclaves that belong to the Netherlands. That is some sick complicated geography. The whole mess is a result of medieval land trades and swaps between noble families. Belgium became independent from the Netherlands in 1839 (Treaty of London). The last no man's land was divided (to Belgium) in the 1950's.
There are houses that are divided by an international border. They are decreed to belong to that state on the side of which their front door is. This has been circumvented by at least one entrepreneur by building another front door. One house has the border going through its front door and thus it has two addresses.