This may have been discussed and/or (ab)used a lot already, but we ran into an interesting event earlier
We decided to show AI War to another friend who'd never played/heard of it, so we started up a 4 player game. He only had the Trial version, so I picked the most 'normal' of the Easy AIs on difficulty 7 (Sledgehammer and Zenith Descendant I think), to avoid playing a match filled with turtles/miners/shielders, and to hopefully have us all die in a hilarious fashion to some mistake he made.
Well, he made a mistake, but not what I was expecting.
He picked up the basics fairly quickly, but during our very first planetary assault, he destroyed a Zenith fleet container. Oops.
He was now -400k or whatever in the red for Energy. Except, he had a giant fleet of Mark IV Zenith ships. And as far as we could tell, while he could no longer build up his empire, he was still perfectly able to rampage with his fleet.
Consequently, we had him simply ravage every world we wanted taken with his massively overpowered fleet, while we built up behind him. No AI world that early in the game could stop the fleet, and at that low of an AI Progress level, the waves sent against us were laughably easy to defeat.
Note that we actually lost this game, but it was mostly due to another newbie mistake he made, I'm pretty sure had that not happened, we would have been knocking on the AI's homeworlds in short order (we had them both scouted, and within a few jumps of our systems, inside about 90 minutes of play!)
You wouldn't be able to get away with this abuse in single player, because you wouldn't be able to build the necessary defenses to protect against AI waves while using the fleet, but in multi, it didn't matter - he killed, we built.