So, going into arena war game with your friends, blinking through the wall. Goin far enough to hit a magic line where he sees you as unfriendly and you see him as friendly, you trade and before you both hit accept on the super expensive item/s you pull your net cable out and BAM you duped the items.
Or what about, trade someone and when you go to walk through an instance portal hit accept you DC, relog and you can then "open" or combine the item and it will never go away, granting you 10000s of items for nothing...
It's in the game, clever use of game mechanics.