Minecraft stops responding or just closes without an error while in parts of the nether. After checking the world in MCEdit, I found it only happened when I was close to multiple ghast fireballs.

Also, while under the fireballs, it was hard to place blocks, the bounding box of blocks flashed, now allowing me to place blocks.

To repeat:

Enter Nether
Get a Ghast to shoot at you
Leave server
Join server
Repeat until you crash