Logging in the specific world crashes the game.

What I expected to happen was...:
that the cheated potion effect (miningfatigue) would be fine with a potency level of 1000 or something.

What actually happened was...:
It... Crashed. And I couldn't log on to that world again.

Steps to Reproduce:
1. Use the /effect command in cheats or SPC to get Mining Fatigue 1000 or something
2. Try to break a block.
3. ...