Take a low low planet count person, have them take as many planets as possible off another player (or more) at the same tick, full build the shit out of them super cheap with 99% con and you avoid the
“Empire Overbuilding Cost”.
Or this statistic:
“Our empire’s large size increases construction costs by 392.80%.”
Then a few ticks later you give them back to the other players and they (if you have the resources) can jump the planets again because their number will be super low too.
There is also one tick of insane research (like res/cash reseach) that is there when you take the planets back because NW hasn’t been applied - and the numbers are crazy high. Eventually that will be exploited as well, but that would be some hard core fucking play by some really active bankers.