Do the milkshake the milkshake do the shake
The nature of the bug was not revealed, but senior community manager Tarib explains it'll take a little while to fix:
We will keep on working over the weekend to fix the remaining issue and make sure that the final version will be ready for the live servers next week. As soon as all the details are available, we will announce the downtime for the update over the usual channels early next week.
Jimmy the Geek