This error is already known to Blizzard and they’re working to fix it as we speak, but this is just a heads up for everyone playing.
The bug occurs early in the game if you equip the Templar follower with a shield. Right now, doing so causes a Error 3006 message and boots the player from the server. According to the Blizzard forums once this happens you can’t log back in until the bug is fixed as it continues to give you the error again and again.
So remember: DON’T GIVE THE TEMPLAR A SHIELD.