Affects Version/s: Minecraft 1.4.1
Fix Version/s: Minecraft 1.4.3
Environment:Java 7 (64 but) update 9.
1. Create a 100 block long rail line.
2. Load a mob into a cart (I did it with a villager but it works with others).
3. Send them down the track.
4. Follow and break the cart.
5. Entity vanishes.
6. Save and reload the world and the entity will re-appear.