[marc_abonce]

The border is a river. But rivers slightly change their course over time.

Croatia claims their original pre-Yugoslavia border, based on the historical course of the river. That's because some of its municipalities follow that old line. Serbia, on the other hand, claims that the current course of the river is the border.

The unclaimed land loophole happens because the curves of the old and new rivers intersect in a zigzag (or DNA-like) manner, so some pieces of land are claimed by both and some pieces are claimed by neither. The lands claimed by both have more value than the lands claimed by neither so neither country takes over those tiny, unpopulated strips of land because doing so would implicitly accept the other country's border claim and therefore serve as a surrender of the better lands to the other country.

Or at least that's my (mis?)understanding, here's more context: https://en.wikipedia.org/wiki/Croatia%E2%80%93Serbia_border_...