Put the summary of the bug you're having here

What I expected to happen was the minecart would slowly descend because of the chicken.

What actually happened was it crashed Minecraft, although I could see the minecart with the chicken. Sometimes it only crashes when you try to get in the minecart
Steps to Reproduce:
1. Open World
2. type in this command: /summon Minecart ~ ~ ~ {Riding:id:Chicken}}
3. If it does not instantly close down, try jumping in the minecart.