I've posted this one before, but it's worth posting again:
I was playing against the AI, (high difficulty and bonus) and after a couple of difficult fights, I was clearly winning the game. (first position, not finding any resistance) So I thought: "lets make this game more interesting"
So I offered the weakest AI player a trade it couldn't refuse: I was willing to trade my whole empire (ships planets everything) in exchange for two random planets. The Idea was to rebuild my empire from those two worlds.
The AI send a message back: " Sure we accept, you get these two worlds (AI specified two worlds) we get your empire" I accept, and am horified when I discover that the two worlds the AI traded me contained no population, (plagued) And thus, with no population, and no production, I had lost the game.
(needless to say I also made the mistake of not saving the game, so there was no way back either)
If this doesn't win me the "most stupid SE4 player award" than I give up..