[Dracus]

In SE3 I would make a treaty with a race, send in a builder ship and build starbases around their homeworlds. Then attack and capture homeworlds.

The AI just pulled this trick on me for the first time ever.

In a conflict with another race, my allie while attacking the enemy, sent a huge fleet to my homeworld to refuel. Since I had asked them to attack a planet in the next system and they said they would.
I figured the fleet was for that attack.
As it turns out as soon as they resupplied off my planet. they sent in a small set of ships to attack the target planet. They then broke the treaty with me and took my homeworld.
I was taken by complete suprise, I had not seen the AI pull this before until I got se4.
But they only did this after I had pulled the same trick in the game eariler on the race we were fighting.