Croatia and Serbia have a similar problem. The border between them was drawn up during the federalisation of Yugoslavia, but the problem lies in that Serbia claims the border follows the modern course of the Danube (thereby giving them more land), whereas Croatia asserts it should follow the original course drawn before the Danube was straightened for ease of navigation.
This leads to one piece of land being claimed by neither. Naturally, this has lead to some chancers to travel to the land and claim it as their own ‘micronation’, such as a Czech man who proclaimed “Liberland” on the territory, although neither Serbia nor Croatia take him seriously.
While not on the topic of unclaimed land, I feel, seen as this is a railway forum, I should mention the peculiar Belgian-German border. After the Great War, the Treaty of Versailles granted some land to Belgium as reparations — one of these is the Vennbahn, a strategic railway line that crossed the border. The Vennbahn’s path would be granted to Belgium, in the process separating land west of the line from the rest of Germany. Although the line is closed now, the irregular border remains.