Sixfeetunder was a player of RuneScape. He was well known during the RuneScape Classic era for finding the Runescape Classic Duplication glitch. As of 9/11/2014 the display name "Sixfeetunder" has been taken by another player.
In 2003 he attempted to trade a Scythe 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 to the public. People made large quantities of rare items, including Partyhats (Mostly Pink partyhats, now known as Purple partyhats), 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.