During the Falador Massacre on 6/6/06, the infamous bug abuser, Durial321, was seen wearing one of these, looted from one of his victims.
As part of the April Fool's Day celebration in 2012, Jagex released an update claiming "Green p-hats ... are lying strewn throughout the streets of Lumbridge, Varrock and Falador." Rather than Green partyhats being dropped, peahats were dropped.
During a small get together by Player Moderators a Duplication method was discovered that has been in game since the 8th of November, 2009, using in-game technology JaGeX was able to track the Party Hats which had only been discovered by a small amount of people, the Party Hats were not removed from the game and therefore left the Price of the Green Partyhat to drop considerably (-32.8%) Grand Exchange average price. The Glitch has since been patched.