Between server outages and game breaking exploits, Blizzard's had its hands full with Diablo III.
Players have recently uncovered a game breaking exploit that, after executing a series of abilities including teleport, make a wizard invulnerable. The exploit is actually pretty simple to execute, without any need for outside programs.
Once players have executed the specific combination of abilities, the wizard will then turn invulnerable, with enemies either avoiding the character or attacking and missing. The exploit isn't restricted to any mode, and has been tested on every difficulty. The only drawback to the exploit is that it locks players out of some of the character's abilities, but invulnerability in Diablo III is a wonderful boon—especially in Hardcore or Inferno—worth trading a few skills for.
The issue's already was made public on YouTube and the Diablo III forums. It's likely that Blizzard will be throwing out a hotfix to lock down the exploit. After all, such an exploit is ripe for abuse to breeze through beating the game.