[tremon]

Borders are now settled once and for all

Not necessarily. For example, The Netherlands and Belgium exchanged parts of lands in 2017 to make their border align again with the Meuse river: https://nos.nl/artikel/2112869-nederland-krijgt-belgisch-sch...

For context: the border originally followed the flow of the river, but a few decades earlier the river was straigthened by dredging to enhance the traffic on the river; the border was left as-is though, leaving both countries with hard-to-reach peninsulas on the "wrong" side of the river.

[jeroenhd]

The Netherlands and Germany are even in a border "conflict" about the exact way the Ems estuary is divided (DE refers to an old document, NL claims the border goes down the middle of the water).

It's not much of a problem in practice because of agreements made decades ago and there is no (or very little) actual land involved, but the borders are still technically disputed by both side.