I'm not sure if this qualifies for a bug - it's just funny AI behaviour in a rare case.
If you attack an AI player VERY soon he will switch to producing Defender units on his planets.
He also does that when he has NO weapon or armor tech whatsoever so he's burning resources on 0/0 ships that serve no purpose whatsoever and will be destroyed every turn.
SOME defense. =P
Granted, by the time you could possibly have a troop transport over his planet he WILL have weapons so *shrug*.
In the current game I started with a precursor library on my home planet and basic mil tech. I raced for space weapons and was able to blockade one computer player on his first 2 planets.
No, I'm not playing nice.