I noticed this bug by spawning in stationary Ghast Fireballs using Command Blocks.

Under some unknown condition (outside of just existing), Ghast fireballs will crash Minecraft almost immediately to a couple seconds.

To reproduce:
1) Start a Creative world with Cheats enabled.
2) Give yourself a command block by chatting /give <username> minecraft:command_block
3) Place it down, and put the following command in it:
/summon Fireball ~ ~2 ~

4) Power the Command Block by placing Redstone torch next to it.
5) Anywhere from immediately to a couple moments, Minecraft crashes with an unexpected error. NullPointerException something or other.

Random information:

• It doesn't matter if the Fireball entity is stationary (ie, the direction tag is set to [0.0,0.0,0.0]) or moving.
• This bug doesn't have anything to do with the Fireballs exploding.
• I've only tested this with Fireballs; I'm not sure if any other entity has this issue.

Attached is the file generated by Minecraft. This bug definitely did not exist in the previous snapshot. This is the first bug report I've submitted ever, so pardon any mistakes in this report