You might want to have a look at the dispute about the border between Germany and the Netherlands in the Ems Dollart estuary. Basically Germany claims the whole river and estuary, the Netherlands claim the border is in the middle of the river. This goes back to a dispute about a deed of enfeoffment granting the whole river and estaury to East Frysia. At the time this was a dispute inside the Holy Roman Emperor and the main dispute is about the deed being backdated and forged. That border later became an international border when the Netherlands became independent, but the dispiute remained the same. There are various treaties regulating who can do what where, whose police is responsible where etc., with an emphasis on friendship and cooperation, but always with the caveat that nothing in those treaties can be constructed as either side giving up their claims...

If you are into 84 page treatises about bizarre border disputes, I'd recommend The Ems-Dollart Predicament