Bobble hat was removed from RuneScape after an update, but still exists in-game for those who had obtained it.
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The Bobble hat was one of the eight receivable hats and scarves from the 2005 Christmas Event. If a player needs a replacement, talk to Diango in Draynor Village. It matches the Bobble scarf. It is a holiday item and purely for decoration. This item is discontinued. Many players mix this hat with the warlock costume as it matches them so closely.
During the 15 Days of Christmas in 2013, this item, along with other christmas-themed equipment, were given the Hybrid style of combat, and stats that scaled with the player's levels. This was so players could use these items in combat to assist in hunting for festive crackers during the event.