Duplication glitch

The Party hat duplication glitch was the most famous glitch of RuneScape Classic, exploited on November 7, 2003. Its effects continue to have an impact on the economy of RuneScape today.

Execution
The glitch was not 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 not duplicate existing items but instead created new ones.

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.

In his excitement, the experimenter revealed to very few people how to execute the glitch. These players then duplicated several items, the most expensive at the time: specifically party hats, rune two-handed swords, and dragon square shields. The most expensive item, the pink party hat, was created most amongst them. That day, though, one of these people revealed that the glitch existed.

Effects
Jagex permanently banned some players who were caught abusing the glitch. Jagex couldn't trace the party hats/crackers, and so only a few accounts with billions of party hats were banned. The only party hats that were lost were the ones on the banned accounts, as no party hats were deleted (this was confirmed in a Jagex News update the day of the event). Jagex did not roll back the servers, so the most expensive party hat dropped to the least expensive. The pink party hat (now the purple party hat) remains the least expensive today, it was the most expensive back then.