Well that's... dumb.
I enlightened a bronze age planet in the center of my empire. They became my protectorate, and the little circle around them became contested territory. I kept the mining stations there, but all the systems in it was marked as unowned. I didn't think much of it until my neighbor popped in and built a frontier outpost in the system (which has 2 of my colonies and the protectorate colony).
The result? The system is now contested by 3 empires, and him having that little blob of their own territory in the center of my empire gave them a big swathe of my territory. The borders that once looked like
X
___________
is now
X
/ \
___/ \____
And all the systems in that area along with all their stations now belong to my neighbor. Including a system with 12 points of science, a wormhole with another 5, a system with a 24 size planet that I was about 3 months away from completing terraforming, and my only source of pitharan dust. All that conquered by building a station without a shot fired while we have a nonaggression pact, and I'm the agreement breaking aggressor if I want to take it back.
I used console commands to destroy the station and closed my borders to him. Now my friendly neighbor is a bit pissed at me, but at least he can't do it again. Glad I'm not playing ironman.