Grand exchange zombie apocalypse

On 10/29/2011, a huge amount of zombie spawned within the Grand Exchange. Players quickly started taking them out, but due to lag, hundreds are there while im typing this article.

It is still unknown what the cause was, as this is just a recent happening. It could be an intentional thing, or, more likely, a mistake. It is not known if it affects other worlds, as the screenshot was taken on world 2.