The Duplication glitch was the most famous glitch of RuneScape Classic, exploited on 7 November 2003. Its effects continue to have an impact on the economy of RuneScape today. Before the glitch, Pink Partyhats (Now called Purple Partyhats) were the most expensive partyhat. Following the glitch and continuing today, they are the least expensive.
The glitch was not originally an attempt to duplicate items. A player named "Sixfeetunder" had been experimenting with RuneScape by using a third-party program called AutoRune in an attempt to simply trade the scythe. Although the player never managed to trade the scythe, he did discover how to create any non-stackable item. Therefore, the glitch did essentially duplicate the item, creating a new one although you didn't even need to own the item to "duplicate" it, so it's more appropriately called "item spawning".
The glitch was performed by trading with a player, and then using AutoRune to enter the numerical value that corresponded with a certain non-stackable item to have an item pop up and be traded to the other person. For example, the number for the Rune 2-handed sword was 81, so that value would be entered into AutoRune and the player who was trading with the one using AutoRune would be able to receive that item. The quantity of the item is shown as 0 on the trade screen of the player creating the item but it would show up as a quantity of 1 on the trade screen of the player receiving the item.
In his excitement, the experimenter revealed to a few people how to execute the glitch. These players then created several items, the most expensive at the time: specifically partyhats. Within days, hundreds of players knew how to execute the glitch.
This led to a crash in the price of many rare items and the effect this has had is still visible to today in certain items. Many people do wonder what the price and abundance of the rare items would be like if the glitch was never found, as the glitch brought in hundreds of illegitimately acquired rare items. Because of the glitch, the purple partyhat remains the cheapest of them all, despite originally being the rarest and most expensive partyhat.
Response by Jagex
Although Jagex soon found out about the glitch, they could not immediately do anything to fix it, as they themselves did not understand the process behind the "duplication". Several days of item duplication made a massive change to the RuneScape economy as players started to duplicate other, less valuable items such as ores. Eventually, Jagex offered a lifetime RuneScape membership for whoever could tell them how the bug worked, with multiple players coming forward and explaining it to them. This allowed for Jagex to disable the glitch. Only the first player to reveal how the glitch worked was rewarded. Although they understood what the long-term effects of the economy in RuneScape would be, Jagex refused to rollback the game.