A couple (fixed) trade/duplication exploits from the past were:
An exploit that allowed you to duplicate items via the guild bank (I don't recall the details). The biggie there was people duplicated Radiant/Ebon crystals and got the best DoN rewards the day of release.
An exploit using insta-clicky potions where, by using some esoteric series of clicks you could turn a No Drop item into a potion, place it in the shared bank, log into another character and find the potion restored to its original staus and withdraw it, thus allowing you to trade No Drops between characters on a single account
An exploit using community tradeskill containers that allowed people to trade No Drop items between characters on different accounts (no idea on the details)