Territorial exchange between the Netherlands and Belgium.
In a signing ceremony attended by their respective royals, Belgium agreed to cede about 35 acres of scenic land by the Meuse River in exchange for about seven acres of land from the Netherlands.
This is why basing your border on a river is a bad idea. The present border follows the old river channel, and as the river shifts and meanders (like a sidewinding snake), it leaves the border all kinds of fucked.