Right after you run an instance or something, if you KNOW something drops off a boss there,(do some research) and it didn't drop, pettion a GM saying it had dropped and you looted it/won roll, and never got the item, if the GM falls for it they should restore the item (That never existed) and you just scammed a GM you cheater!
For best results, id say dont take the "best" loot off the mob it drops from, then it seems more beleivable to the GM, and reset the instances before petitioning, just to be safe.
Also, take note this is more risky then most scams, seeing as you are scamming a GM, and if caught, they might... take action ...you dont like !!