Geo, I have noticed this too, but with some other observations. The planet selected for colonization is already colonized by another race. You can't see them, they can't see you but the colonize order knows that that planet is invalid due to the fact that it is already colonized....at least this is what I think is happening sometimes.(I can do this sometimes at the start of a game with the 'ancient ability, find what looks like the other races homeworlds and then attemp to colonize, if fails then that is where they are.) Either way it is a bug, your ship should have to go there to find out it is colonized....(or maybe I just imagined this whole scenario....damn, 4:20am, time for bed....
)
------------------
"And what the hell would you know about sanity?" demanded Beatrice. "There are depressed lemmings on the edge of cliffs who've got a better grasp on reality than you have. And more common sense."--Simon R. Green 'Deathstalker Rebellion'.
"We are all...the sum of our scars"....(paraphrased) Matt. R. Stover-'Blade of Tyshalle'.
"Memory," the old man said softly, "is tricky. I have learned two things from it. One: That without meaningful memories, there is no life. Two: that we are desperate for our lives to make sense, to have meaning,--and at a great enough remove , all memory is malleable." -Michelle West 'Sea of Sorrows', SunSword book IV.