Stealth guard posts and AI Eyes, I find these unfair, you can probably guess why.
Late in my game so my firepower took care of the Eye easily, but still I think stealth guard posts should not be on the same planets as AI Eyes. Since it's simply just not fun that you have to look for something you can't see nor can you have the slightest clue of it's whereabouts.
Yesterday when I ran into this I almost reported it as a bug since I couldn't see any remaining guard posts and the Eye was still there:)