This is rather unexpected... If an egg is thrown, the projectile entity simply isn't serialized. This causes the egg to be deleted when the world is reloaded, and also causes Egg spawners (which, if memory serves, used to work fine) to crash the game.

Steps to Reproduce:

1. Throw several eggs in the air.
2. Before they land or collide, save and quit the world.
3. Reload the world, and observe that the eggs are gone.