Update: Old School Runescape is back online following a 20-minute server rollback. In other words, all of the gold generated by the bug has vanished as quickly as it appeared. Additionally, mischievous players who purchased in-game bonds using ill-gotten gold and redeemed them for membership have had their 'bonus' membership revoked and "will also see action taken against their accounts and will temporarily be removed from the game." Everyone else should be in the clear, though Jagex advises some players "may be affected by an issue attributed to their save files which will prevent them from logging in," which the studio is still working to resolve.
In a forum post, Jagex explained what caused the billionaire bug:
One of the team was implementing the code for the pickpocketing update. This involved producing stackable pouches that are treated as being coins. On death, the aim was to convert them into real coins, partly so that a PKer could get the value if applicable.
When converting a stack of items into a stack of coins, it's appropriate to check that the calculation doesn't overflow the max integer limit of the game's language (2.1 billion). Unfortunately, the logic used for this calculation was incorrect, and when executed on stacks of other items (not the pouch itself) the result was to convert the stack to 2.1b coins.
Regrettably, although the pouches were found to be acting correctly during testing, the flaw affecting other items was not spotted as it was not expected to have changed.
Every Old School Runescape player dreams of one day attaining the most gold you can possibly carry: 2.147 billion, the fabled "max cash." Due to a bug introduced in an update this morning, for many players that dream suddenly came true: countless OSRS players received max cash stacks from sources which would normally yield a few thousand gold. As a result, roughly 20 minutes after the update went live, developer Jagex outright turned the game off, temporarily disabling all servers while they look into the gold duplication bug.
One Jagex developer, who goes by the mod handle Mod Ash, discussed the issue on Twitter, confirming that "max cash stacks" were "being generated on death if you had certain items." Mod Ash also said the OSRS team is working on a rollback—that is, reverting the game to its state before the bugged update, effectively undoing all the duplicated gold as well as everything else that happened in the 20 chaotic minutes following the update.
Ironically, the update that caused the bug, a "game integrity" update of all things, was a change to the Thieving skill intended to combat bots and gold farmers. As YouTuber OakDice pointed out, some wires got crossed and this update caused instanced events that store or generate gold to yield billions. You can see the bug in action in this clip from Twitch streamer Purpp, who was understandably surprised to become an in-game billionaire:
OSRS is still offline at the time of writing, but Jagex says it is working on a fix. In a statement to PC Gamer, Jagex said:
"Old School RuneScape was taken offline by Jagex this morning at 11.49am BST, just 19 minutes after the update that introduced the bug went live. A rollback, the game’s first, is planned to take place in the next few hours to rectify the situation, taking it back to just before Old School RuneScape was updated. We do not have an estimate for the amount of gold generated during the time the bug was live, however the imbalance should be fully resolved when the rollback is complete later today."