A loophole has reportedly been found in Diablo III that can, in effect, make you invincible. Which for a game that has a real-money auction house attached to it could be a bit of a problem.
While it only works for the Wizard class, there's apparently no restriction on which game mode (normal or harder) you use. The exploit renders a few effects inactive, but on the flipside, well, you'll be invulnerable.
A poster on the Blizzard forums says to active the exploit, you need to do the following:
Step 1: Select Teleport - Fracture. Bind it to a key
Step 2: Select Archon, tested with Improved Archon
Step 3: Hover your mouse over or near your charcter
Step 4: Press Teleport
Step 5: QUICKLY(!) Press Archon
Note that, given the consequences of letting people run around without fear of dying while picking up items they can sell for actual money, Blizzard will likely be working super fast to get this fixed.
EXPLOIT/BUG: INVULNERABLE WIZARD [Blizzard Forums]
Gamebreaking Godmode Bug. Oh wait, game's already broken anyway [Chango99]
GOD MODE - Be Totally Invulnerable Forever - Diablo 3 Wizard Exploit/Bug [YouTube, via NeoGAF]