The iron pickaxe head was part of a discontinued random event, in which the pickaxe head flew off the handle while the player was mining. To fix it, the player had to collect it off the ground and reattach it to the handle. A similar thing used to happen to players while Woodcutting, only with an axe head and axe handle. Note that it was usually cheaper to buy a new pickaxe than the pickaxe head. In 2007, Jagex removed this random event and all pickaxe heads were removed from the game.
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