The coin pouch glitch was a major glitch that occurred after a game update on 12 July 2018.
The code which handles death in game was changed in the update in order to accommodate the coin pouch[1], but was done incorrectly[2] and wasn't set up to handle stacks of other items[3], which caused certain items in a player's inventory to become a stack of 2,147,483,647 coins. 2,147,483,647 is the maximum value a 32-bit signed integer can take, which is what data type RuneScape uses for item quantity.
Jagex immediately took action by taking the Old School servers offline on 11:54 a.m. (UTC) after announcing to do so five minutes earlier[4], around 20 minutes after the update went live. Hours after the glitch occurred, Jagex announced they were performing restoration (a rollback) of the game.[5][6]
After the rollback was finished and the servers were made online again, Jagex said they would be removing illegitimately acquired membership from accounts that was from purchasing bonds via money produced from the glitch. They also said those who abused it would be temporarily removed from the game.