This issue has been reported prior however was not fully described.
When spawning a Ghast with a spawn-egg near the 30mil farlands border, the game will become unresponsive.
It appears the client side continues to operate (ghast tentacles move, animated textures still animate, etc.) however all mobs freeze and the server no longer commits to disk.
After reloading the world the ghast is no longer present, and all changes between the last commit to disk and lock-up are reverted.
Note: Cannot reproduce with /summon