The trade bug/exploit lets anyone modify a trade upon being offered one and then accept it.
In other words, if you trade embassy with me, I can take all your gold per turn, cities (save capital) and all lux/strat ressources.
Player A trades embassy with player B
Player B clicks "Modify" and changes the trade (add gpt, lux, cities, whatever)
Player B close the window
Player B reopens the window with the pop-up (notify) icon that player A's trade's made appeared
Player B accepts trade
Now player B has everything player A possesses. Also, unless player A noticed the trade being modified, he'll have no way but to declare war to retake ressources. Cities are, however, gone forever.