item through a variety of different means and ranging from "Normal" items used as the stock weapons of each class, to "Unique" items used as the base obtainable items from the item drop or achievement systems, to far rarer qualities such as "Strange", "Unusual" or "Decorated" which feature special cosmetic effects that can immensely increase the market value of a given item; Strange items keep track of kills or other objectives achieved while equipped in-game while Unusual items feature item-specific particle effects, with both Strange and Unusual items being obtainable through rare crafting items or randomly obtained in place of the far more common Unique items. Decorated items are instead redeemed from rare items known as "war paints", awarding the player a weapon retextured with a pseudo-random cosmetic skin. Other qualities include "Vintage", awarded to older items to compensate for changes in obtainability, and "Collector's", created through combining 200 Unique instances of a single item. Cosmetics and war paints are typically released through seasonal "cases" that award a random item from an associated collection unique to the given season of a specific year. Such items are additionally assigned a "grade" from "Civillian" to "Mercenary" to track their relative rarity within a collection. Third-party websites such as the crowd-sourced backpack.tf have been created to aid users in trading, as well as track the value of in-game items. Crate keys, crafting metal, and in-game items such as an "earbuds" cosmetic (also referred to as "buds") are all used as currency due to their value. The economy of Team Fortress 2 has received significant attention from economists, journalists, and users, due to its relative sophistication and the value of many of its in-game items. It has often been the subject of study. It operates on a system of supply and demand, barter, and scarcity value, akin to many real-world economies such as that of the United States. In 2011, it was reported that the economy of Team Fortress 2 was worth over US$50 million. 2019 Crate bug On July 25, 2019, a bug was mistakenly included in an update - if players unboxed certain older series of Crates, they would be guaranteed to receive an Unusual-grade cosmetic item, compared to the usual 1% chance of obtaining an Unusual-grade cosmetic item from a Crate. This damaged the in-game economy, causing Unusual-grade cosmetic items able to be unboxed from these Crates to drop substantially in