Right after the NA servers came up, Blizzard pushed the announced addon API changes to Action Bar addons due to things that could break the secret value system. These changes were never tested and were only pushed to live servers meaning that addon authors had no chance to fix these early. Due to this, many players had their addons broken mid dungeon run, and now need to wait for addon authors to update their addons due to this last minute push. Note: This is only going to happen in combat.
Known affected addons:
ElvUI - Action Bars don't work
Bartender - Action Bars no longer accurately show cooldown.
Please be kind to your addon authors, this last minute change is not their fault. They are surely working hard to fix their addons.
