Perhaps Blizzard overreached with Diablo III. In development for over four years (and possibly as far back as 2001), the game is more ambitious than anything the series has seen before.
Naturally, that has lead to a variety of error messages plaguing players on the game’s launch day.
Blizzard currently lists five errors in the game’s official forum, with Error 3006 being the most serious. Joystiq describes it as a game-breaking bug that occurs if a player swaps an equipped item with a Templar. After the error occurs, players are unable to log back in to Battle.net, even if they restart the game.
Blizzard is looking into to these errors and will hopefully have a fix ready soon.