Sixfeetunder was a player of RuneScape. In 2003 he attempted to trade a Scythe (untradeable holiday item) using a macro called AutoRune but it didn't work. Instead he found out how to create items from nothing. The amount being given would show as 0 to the player giving the item and 1 to the player receiving the item. He told around 3 of his friends how to abuse the bug and it eventually leaked, reaching Jagex. He and they made large quantities of the rarest party hats (Pink at the time, now Purple), which made the price of that partyhat crash as there was a large supply. Apparently Jagex offered free lifetime membership to whoever told how to reproduce the bug , but instead they banned everyone involved in it, including other accounts from the players who hadn't logged in at the time.
As of 9/11/2014 the RSN "Sixfeetunder" has been taken by another player.